BIOL300: Chapter 17

chapter contents

the cytoskeleton

intermediate filaments

microtubules

actin filaments

muscle contraction

cytoskeleton gives a cell

the cytoskeleton gives a cell its shape and allows the cell to organize its internal components and to move;

the three types of protein filaments that form the cytoskeleton differ in their composition, mechanical properties, and roles inside the cell

intermediate filaments

intermediate filaments are ropelike fibers with a diameter of about 10 nm; they are made of fibrous intermediate filament proteins;

one type of intermediate filament forms a meshwork called the nuclear lamina just beneath the inner nuclear membrane;

other types extend across the cytoplasm, giving cells mechanical strength and distributing the mechanical stresses in an epithelial tissue by spanning the cytoplasm from one cell-cell junction to another;

intermediate filaments are very flexible and have great tensile strength; they deform under stress but do not rupture;

see diagram (spaghetti-looking) and micrograph (looks like a plaid pattern) of intermediate filaments

microtubules

microtubules are hollow cylinders made of the protein tubulin; they are long and straight and typically have one end attached to a single microtubule-organizing center called a centrosome;

with an outer diameter of 25 nm, microtubules are more rigid than actin filaments or intermediate filaments, and they rupture when stretched;

see diagram (spider-looking) and micrograph (looks like dark parallel lines) of microtubules

actin filaments

actin filaments (aka microfilaments) are helical polymers of the protein actin; they are flexible structures, with a diameter of about 7 nm, that are organized into a variety of linear bundles, 2D networks, and 3D gels;

although actin filaments are dispersed throughout the cell, they are most highly concentrated in the cortex, the layer of cytoplasm just beneath the plasma membrane;

see diagram (lots of dense lines around outside of the cell) and micrograph (beaded chain of a necklace) of actin (micro)filaments

overview of the physical properties and functions of the 3 cytoskeletal systems in animal cells

microfilaments -

actin binds ATP --> form rigid gels, networks, and linear bundles --> regulated assembly from a large number of locations --> highly dynamic --> polarized --> tracks for myosins --> contractile machinery and network at the cell cortex;

microtubules -

alpha-beta-tubulin bind GTP --> rigid and not easily bent --> regulated assembly from a small number of locations --> highly dynamic --> polarized --> tracks for kinesins and dyneins --> organization and long-range transport of organelles;

intermediate filaments -

IF subunits don't bind a nucleotide --> great tensile strength --> assembled onto pre-existing filaments --> less dynamic --> unpolarized --> no motors --> cell and tissue integrity;

see and memorize diagram!!!

the cytoskeleton - essential concepts

(1) the cytoplasm of a eukaryotic cell is supported and organized by a cytoskeleton of intermediate filaments, microtubules, and actin filaments;

(2) prokaryotes contain proteins that are analogous to tubulin, actin, and intermediate filaments and play roles in cell shape, division, protection, and polarity

intermediate filaments - introduction

intermediate filaments form a strong, durable network in the cytoplasm of the cell;

contain bundles of intermediate filaments that gather at a desmosome to connect two cells;

see micrographs and Movie 17.1 Intermediate filaments

intermediate filaments - major concepts

IFs are strong and ropelike;

IFs strengthen cells against mechanical stress;

the nuclear envelope is supported by a meshwork of IFs

IFs are strong and ropelike

IFs are like ropes made of long, twisted strands of protein;

(A) alpha-helical region of monomer;

(B) coiled-coil dimer of 48 nm length;

(C) staggered tetramer of two coiled-coil dimers;

staggered tetramer in bulk --> lateral association of 8 tetramers --> addition of 8 tetramers to growing filament;

see micrograph of spaghetti-like IFs and diagrams

IFs strengthen cells against mechanical stress

IFs are divided into four major classes:

cytoplasmic --> (1) keratin filaments in epithelial cells, (2) vimentin and vimentin-related filaments in connective-tissue cells, muscle cells, and glial cells, and (3) neurofilaments in nerve cells (amyotrophic lateral sclerosis);

nuclear --> (4) nuclear lamins in all animal cells

a mutant form of keratin makes skin more prone to blistering

see micrographs of normal mouse (A) versus blistered mouse (B);

disease - epidermolysis bullosa simplex

plectin

plectin aids in the bundling of IFs and links these filaments to other cytoskeletal protein networks;

see micrograph

the nuclear envelope is supported by a meshwork of IFs

IFs support and strengthen the nuclear envelope;

magnifying a section of nucleus membrane -

nuclear envelope, nuclear pore, nuclear lamina like spaghetti noodles lining the inside of the nuclear envelope, and chromatin is numerous spirals within the nuclear;

see diagram and micrograph of very intricate and clean meshwork

defects in a nuclear lamin

defects in a nuclear lamin can cause a rare class of premature aging disorders called progeria;

disease - hutchinson gilford progeria syndrome

intermediate filaments - essential concepts

(1) IFs are stable, ropelike polymers - built from fibrous protein subunits - that give cells mechanical strength; some IFs form the nuclear lamina that supports and strengthens the nuclear envelope; others are distributed throughout the cytoplasm

microtubules - introduction

microtubules usually grow out from an organizing center;

see (A) fluorescence microtubule micrograph, (B) nondividing cell with a centrosome (where microtubules originate in animal cells), (C) dividing cell with poles of mitotic spindle (where microtubules originate) and contain centrioles, and (D) ciliated cell with cilia protruding from top of cell and basal bodies where the microtubules are attached and extend through the cilia

microtubules - major concepts

MTs are hollow tubes with structurally distinct ends;

the centrosome is the major MT-organizing center in animal cells;

growing MTs display dynamic instability;

dynamic instability is driven by GTP hydrolysis;

MT dynamics can be modified by drugs

MTs are hollow tubes with structurally distinct ends

MTs are hollow tubes made of globular tubulin subunits;

tubulin heterodimer = the MT subuni, with a beta on top and alpha on the bottom;

the protofilament is made of attached beta-alpha subunits, and has a plus end and a minus end;

many protofilaments align length-wise to create a cylinder, which contains a (hollow) lumen down the center;

see diagrams and micrographs of (1) MT looking through the down the lumen and (2) microtubule lengthwise viewed from the outside

the centrosome is the major MT-organizing center in animal cells

tubulin polymerizes from nucleation sites on a centrosome;

a centrosome contains a centrosome matrix, many nucleating sites (denoted by miniature orange circles, called gamma-tubulin ring complexes), and a pair of centrioles;

microtubules grow at their plus ends from the gamma-tubulin ring complexes of the centrosome, meaning the minus end is located where the MT attached to the nucleating site; the plus ends are always distal to the centrosome, minus ends attached to centrosome;

see diagrams and fluorescence micrograph with red-labeled gamma-tubulin ring complexes;

see Movie 17.2 Dynamic instability of MTs

growing MTs display dynamic instability

each MT grows and shrinks independently of its neighbors;

some MTs will be longer or shorter than others, growing and shrinking to different lengths from the centrosome; some shrink back rapidly, a behavior known as dynamic instability;

see diagrams

selective stabilization

the selective stabilization of MTs can polarize a cell;

(A) nuclear with nearby centrosome with growing and shrinking MTs;

(B) microtubule capping proteins are shown, with growing microtubules;

(C) the capping proteins can capture the growing microtubules and anchor them;

(D) stable MTs are shown anchored to MT capping proteins and unstable microtubules are still growing and shrinking;

the anchoring contributes to the polarization of the cell;

see diagram

dynamic instability is driven by GTP hydrolysis

GTP hydrolysis controls the dynamic instability of MTs;

growing microtubule -

tubulin dimer with bound GTP (GTP-tubulin) --> GTP-tubulin dimers add to growing end of MT --> addition proceeds faster than GTP hydrolysis by the dimers --> GTP cap results with red-marked molecules, still growing;

shrinking microtubule - protofilaments containing GDP-tubulin peel away from the MT wall --> you get these frayed ends where the ends are peeling away --> GDP-tubulin is released to the cytosol --> GDP-tubulin subunits are floating away from the frayed ends

see diagrams and Movie 17.3 Microtubule dynamics in vivo

MT dynamics can be modified by drugs

drugs that affect MTs -

MT-specific drugs -- action;

taxol -- binds and stabilizes MTs;

colchicine, colcemid -- binds tubulin dimers and prevents their polymerization;

vinblastine, vincristine -- binds tubulin dimers and prevents their polymerization

microtubules - major concepts (part 2)

MTs organize the cell interior;

motor proteins drive intracellular transport;

MTs and motor proteins position organelles in the cytoplasm;

cilia and flagella contain stable MTs moved by dynein

MTs organize the cell interior

MTs guide the transport of organelles, vesicles, and macromolecules in both directions along a nerve cell axon;

along the axon of the nerve cell are microtubules extending lengthwise between nucleus/nerve cell body and axon terminal --> outward transport of red molecules to axon terminal along the microtubules (minus to plus) and backward transport of blue molecules to cell body along the microtubules (plus to minus);

see diagram and micrographs of sequence of five frames of organelles moving rapidly and unidirectionally in a nerve cell axon

motor proteins drive intracellular transport

both kinesins and dyneins move along MTs using their globular heads;

the globular heads make contact with the outside of the MT, and the globular head is the sight of hydrolysis;

dyneins move from plus end to minus end, and kinesins move from minus end to plus end;

the two globular heads of kinesins and dyneins move like they are actually walking along the surface of the MT; the back head will detach and swing around in front of the front head, and repeat; type of motion called processive;

see diagrams

different motor proteins

different motor proteins transport different types of cargo along microtubules;

kinesins have the pair of globular heads, a tail, and at the tip of the tail is cargo, moving from minus end to plus end;

dyneins have the pair of globular heads, a tail, and at the top of the tail is cargo, moving from the plus end to the minus end along the surface of MT;

see diagram, Movie 17.4 MT and ER dynamics, and Movie 17.5 Organelle movement on MTs

MTs and motor proteins position organelles in the cytoplasm

MTs help position organelles in a eukaryotic cell;

(A) diagram of cell with microtubules protruding from the centrosome, with MTs denoted as green, lining the ER denoted as blue;

(B) blue and green fluorescence micrographs of ER and MTs;

(C) centrosomes fluorescence micrographs;

see diagram

cilia and flagella contain stable microtubules moved by dynein

kinesin causes MT gliding in vitro;

a microtubule where kinesin has been fixed and is sliding;

a single molecule of kinesin moves along a MT; from minus end to plus end, five sequences are shown in drawings, showing the kinesin globular heads' "walking" motion along the surface of the MT;

head binding along MT causes ADP to be released from the head and ATP to come into the head; ATP hydrolysis takes place when the head with ATP becomes the lagging head;

see photos and diagram, and Movie 17.6 Kinesin (catalytic core - the globular heads, and neck linker - connects the heads);

many hairlike cilia

many hairlike cilia project from the surface of the epithelial cells that line the human respiratory tract;

a cilium beats by performing a repetitive cycle of movements, consisting of a power stroke followed by a recovery stroke;

flagella propel a cell through fluid using repetitive wavelike motion;

see micrograph of surface of the epithelial cells with numerous cilira, see diagram of power stroke, and see micrograph of flagellum

microtubules in a cilium or flagellum are arranged in a "9+2" array

(A) an array of microtubules, shown in micrograph;

(B) an array of microtubules, shown in diagram;

the 9 are arranged in a circle on the outside, encircling the two in the center;

the diagram shows a radial spoke that protrudes from circle toward the center, the inner sheath which is the circle of the two inner MTs, the central singlet microtubule which is one of the two inner MTs, and plasma membrane surrounding the entire array, A microtubule and B microtubule which form a pair called the outer doublet microtubule (there are 9 of these), the inner dynein arm which protrude from the doublets, and outer dynein arm which also protrude from the doublets, and the nexin which connect the 9 doublets of the outer circle;

see diagram and micrograph

the movement of dynein causes the flagellum to bend

in isolated doublet MTs: dynein produces microtubule sliding when ATP is added; the right-side plus end moves up and the left-side minus end moves down;

in a normal flagellum: dynein causes MT bending; linking proteins are located every so often along the flagellum, and the moving of the dynein causes antisymmetry and bending occurs;

see diagrams

microtubules - essential concepts (part 1)

(1) microtubules are stiff, hollow tubes formed by globular tubulin dimers; they are polarized structures, with a slow-growing minus end and a fast-growing plus end

(2) MTs grow out from organizing center such as the centrosome, in which the minus ends remain embedded

microtubules - essential concepts (part 2)

(3) many MTs display dynamic instability, alternating rapidly between growth and shrinkage; shrinkage is promoted by the hydrolysis of the GTP that is tightly bound to tubulin dimers, reducing the affinity of the dimers for their neighbors and thereby promoting MT disassembly

(4) MTs can be stabilized by localized proteins that capture the plus ends, thereby helping to position the MTs and harness them for specific functions

microtubules - essential concepts (part 3)

(5) kinesins and dyneins are MT-associated motor proteins that use the energy of ATP hydrolysis to move unidirectionally along MTs; they carry specific organelles, vesicles, and other types of cargo to particular locations in the cell

(6) eukaryotic cilia and flagella contain a bundle of stable MTs; their rhythmic beating is caused by bending of the MTs, driven by the ciliary dynein motor protein

actin filaments - introduction

actin filaments allow animal cells to adopt a variety of shapes and perform a variety of functions;

(A), (B), (C), and (D) show different locations of actin filaments in different cells (lining the cilia, stretching across borders, making a circle between a dividing cell);

see diagrams

actin filaments - major concepts

AFs are thin and flexible;

actin and tubulin polymerize by similar mechanisms;

many proteins bind to actin and modify its properties;

a cortex rich in AFs underlies the plasma membrane of most eukaryotic cells;

cell crawling depends on cortical action;

actin associates with myosin to form contractile structures;

extracellular signals can alter the arrangement of AFs

AFs are thin and flexible

AFs are thin, flexible, protein threads;

(A) actin monomer (subunit), an actin filament has a minus end and plus end with many many attached actin filaments;

see diagrams and micrographs of AF

actin and tubulin polymerize by similar mechanisms

ATP hydrolysis decreases the stability of the actin polymer;

actin with bound ADP --> ADP leaves and ATP comes in --> actin with bound ATP attaches to the actin filament opposite the ADP end;

actin with bound ATP is hydrolyzed, and becomes actin with bound ADP;

see diagram

treadmilling of AFs

treadmilling of AFs and dynamic instability of MTs regulate polymer length in different ways;

treadmilling -

an ATP actin monomer adds on to the plus end of the AF, while a ADP actin monomer leaves; continuation of this causes the monomers to cycle down the AF;

dynamic instability -

GTP-tubulin adds to plus end of MT, and rapid growth then loss of GTP cap causes a catastrophic shrinkage, and finally GTP cap is reestablished and the cycle repeats;

see diagrams

drugs that affect AFs

actin-specific drugs -

phalloidin - binds and stabilizes filaments;

cytochalasin - caps filament plus ends, preventing polymerization there;

latrunculin - binds actin monomers and prevents their polymerization

many proteins bind to actin and modify its properties

actin-binding proteins control the behavior of AFs in vertebrate cells;

in the center of the diagram are actin filaments,

<---> actin monomers

actin monomers --> nucleating protein;

actin monomers <---> monomer-sequestering protein;

actin filaments --> bundling protein (in filopodia)

--> myosin motor protein

--> side-binding protein

--> capping (plus-end-blocking) protein

--> cross-linking protein (in cell cortex)

--> severing protein;

see diagram

a cortex rich in action filaments underlies the plasma membrane of most eukaryotic cells

just know this

cell crawling depends on cortical actin

forces generated in the actin-filament-rich cortex help move a cell forward;

actin cortex with lamellipodium is located along the substratum --> actin polymerization at plus end protrudes lamellipodium, causing the cortex to be under tension and also the movement of unpolymerized actin --> myosin motor proteins slide actin filaments, causing contraction, and attachment of the lamellipodium takes place at focal contacts (containing integrins) --> further protrusion takes place moving the cell down the substratum;

see Movie 17.7 Neutrophil chase, diagram

actin filaments allow animal cells to migrate

AFs include lamellipodia and filopodia, which are protrusions from the back end of an animal cell and allows the cell to migrate;

see diagram and micrograph

a web of polymerizing actin filaments pushes the leading edge of a lamellipodium forward

micrograph shows polymerization of new AFs pushing the plasma membrane forward, very very dense, tons of thin lines;

newly polymerized ATP-AFs are pushing the plasma membrane forward along the leading edge of the cell;

the ARP complex and capping protein are involved in the anchoring and elongation process of the ADP-AFs located away from the plasma membrane, consequently lengthening the filaments by adding actin monomers;

see diagram and micrograph

listeria monocytogenes

listeria monocytogenes can "ride" actin bundles out of an infected cell;

polymerization from one end of an actin bundle provides the force that propels a 2.5 micrometer long listeria bacterium through the cell surface; literally is moving with a stream of actin propelling it from behind;

see Movie 17.11 Listeria monocytogenes hijacks the host cell's actin system

actin associates with myosin to form contractile structures

Myosin-I is the simplest myosin;

myosin-I is ~70 nm lengthwise, with a head domain and a tail (looks like a sperm cell);

the myosin-I molecules attach to the actin (the head binds to the actin filament) and driven by motor, they hinge and move, consequently moving the AFs and causing the AFs to slide along the plasma membrane, with the minus end the front end of movement;

they may also have a vesicle attached and can move the vesicle along the AF, from the minus end to the plus end;

myosin-I attached between plasma membrane and AF or between vesicle membrane and AF;

see Movie 17.8 Crawling actin and diagrams

extracellular signals can alter the arrangement of actin filaments

activation of Rho family GTPases can have a dramatic effect on the organization of AFs in fibroblasts;

(A) unstimulated fibroblast cell, looks like a stretched out web;

(B) Rho activation, looks like stretched out sheet of threads;

(C) Rac activation, looks like a popped circular balloon (giant lamellipodia around the cell);

(D) Cdc42 activation, looks like an irregular sun with sun-rays extending out around surface;

all of these involved in regulation;

see photos

actin filaments - essential concepts (part 1)

(1) AFs are helical polymers of globular actin monomers; they are more flexible than MTs and are generally found in bundles or networks

(2) like MTs, AFs are polarized, with a fast-growing plus end and a slow-growing minus end; their assembly and disassembly are controlled by the hydrolysis of ATP tightly bound to each actin monomer and by various actin-binding proteins

actin filaments - essential concepts (part 2)

(3) the varied arrangements and functions of AFs in cells stem from the diversity of actin-binding proteins, which can control actin polymerization, cross-link AFs into loose networks or stiff bundles, attach AFs to membranes, or move two adjacent filaments relative to each other

(4) a concentrated network of AFs underneath the plasma membrane forms the bulk of the cell cortex, which is responsible for the shape and movement of the cell surface, including the movements involved when a cell crawls along a surface

(5) AF - 7 nm, MT - 25 nm, IF - 11 nm

muscle contraction - major concepts

muscle contraction depends on interacting filaments of actin and myosin;

AFs slide against myosin filaments during muscle contraction;

muscle contraction is triggered by a sudden rise in cytosolic Ca2+;

different types of muscle cells perform different functions

muscle contraction depends on interacting filaments of actin and myosin

myosin-II molecules can associate with one another to form myosin filaments;

(A) myosin-II molecule contains two heads and two tails twisted tightly together; length is 150 nm;

(B) myosin-II filament contains tons of myosin heads and a bare region in the center, the myosin-II tails are all aligned parallel in bulk; length is 1 micrometer;

see diagram

a small, bipolar myosin-II filament

a small, bipolar myosin-II filament can slide two actin filaments of opposite orientation past each other;

opposite end/opposite side heads of the myosin-II will be attached to two parallel actin filaments, and they will slide the minus ends of the AFs past each other;

see diagram

AFs slide against myosin filaments during muscle contraction

a skeletal muscle cell is packed with myofibrils;

the myofibrils are long tubular structures running the full length of the muscle cell, and there are nuclei along the surface of the cell; circular striations are located along the full cylindrical face of the cell;

sarcomeres are the building blocks of the muscle; the sarcomere is the fundamental unit of the muscle, they are attached to each other, and the myofibrils get shorter because the individual sarcomeres get shorter;

see diagram and micrograph

sarcomeres are the contractile units of muscle

sarcomeres are the contractile units of muscle;

(A) Z-disc, myofibrils, and overlap region are denoted on a micrograph of a sarcomere, which is ~2.2 micrometers long;

(B) parallel thick filaments of myosin-II are located between parallel thin filaments of actin; the Z-disc is the attaching site between neighboring parallel sets of actin thin filaments, and the overlap region happens when the myosin-II contracts and the thin filaments slide past one another;

see diagram

muscles contract by a sliding-filament mechanism

muscles contract by a sliding-filament mechanism in the myofibrils;

(A) relaxation -

the myosin filaments are relaxed and the actin filament ends are located far away from each other (no overlap);

(B) contraction -

the myosin filaments move the actin filament ends toward each other, literally pulling the minus ends toward each other within the center of the sarcomere; thus decreasing the distance between the AF ends and decreasing the distance between neighboring Z-discs;

contraction occurs over a few nanometers (relatively not much shorter than when relaxed);

see diagrams and Movie 17.9 Skeletal muscle contraction

the head of a myosin-II molecule walks along an AF

the head of a myosin-II molecule walks along an AF through an ATP-dependent cycle of conformational changes;

begins in the attached state, moves to a released state, then cocked state, moving to force-generating state, and then attached again;

attached-released-cocked-force-generating-attached;

see diagram

the head of a myosin-II molecule walks along an AF - attached

at the start of the cycle shown in this figure, a myosin head lacking a bound ATP or ADP is attached tightly to an actin filament in a rigor configuration (so named because it is responsible for rigor mortis, the rigidity of death); in an actively contraction muscle, this state is very short-lived, being rapidly terminated by the binding of a molecule of ATP to the myosin head;

see diagram

the head of a myosin-II molecule walks along an AF - released

a molecule of ATP binds to the large cleft on the "back" of the myosin head (that is, on the side furthest from the AF) and immediately causes a slight change in the conformation of the domains that make up the actin-binding site; this reduces the affinity of the head for actin and allows it to move along the filament (the space drawn here between the head and actin emphasizes this change, although in reality the head probably remains very close to the actin);

see diagram

the head of a myosin-II molecule walks along an AF - cocked

the cleft closes like a clam shell around the ATP molecule, triggered a large shape change that causes the head to be displaced along the AF by a distance of about 5 nm; hydrolysis of ATP occurs, but the ADP and inorganic phosphate produced remain tightly bound to the myosin head;

see diagram

the head of a myosin-II molecule walks along an AF - force-generating

a weak binding of the myosin head to a new site on the AF causes release of the inorganic phosphate produced by ATP hydrolysis, concomitantly with the tight binding of the head to actin; this release triggers the power stroke - the force-generating change in shape during which the head regains its original conformation; in the course of the power stroke, the head loses its bound ADP, thereby returning to the start of a new cycle;;

see diagram

the head of a myosin-II molecule walks along an AF - attached again

at the end of the cycle, the myosin head is again bound tightly to the actin filament in a rigor configuration; note that the head has moved to a new position on the AF, which has slid to the left along the myosin filament;

see diagram and Movie 17.10 Myosin

muscle contraction is triggered by a sudden rise in cytosolic Ca2+

T tubules and the sarcoplasmic reticulum surround each myofibril;

the myofibril in cylindrical in the center, surrounded by transverse (T) tubules formed from invaginations of the plasma membrane, and sarcoplasmic reticulum creating a web-like covering around the myofibril, and a plasma membrane encloses all of it;

see diagram and micrograph

skeletal muscle contraction is triggered by

skeletal muscle contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum into the cytosol;

an inactive T-tubule membrane with Ca2+ ions filling the lumen of the T-tubule (extracellular space, and there is a voltage-gated Ca2+ channel along the T-tubule membrane, then adjacent to this is a Ca2+-release channel located along the plasma membrane of the sarcoplasmic reticulum, which also contains Ca2+ ions within the lumen of the sarcoplasmic reticulum; the space between membranes (the space of cytosol between the two channels) is about 35 nm);

when an action potential occurs ---> many Ca2+ ions exit the lumen of sarcoplasmic reticulum as the Ca2+-release channel opens, and a couple Ca2+ ions exit the lumen of T-tubule through an open voltage-gated Ca2+ channel;

this movement of Ca2+ into the cytosol outside these membranes causes the myofibril contraction;

see diagram

skeletal muscle contraction is controlled by

skeletal muscle contraction is controlled by tropomyosin and troponin complexes;

along the actin filament of adjacent actin monomers, twisting lengthwise around the actin filament is tropomyosin, and a troponin complex (I, C, T) are located around the filament every few monomers, about 10 nm apart;

tropomyosin blocking myosin-binding sites are located on actin monomers; when Ca2+ is present, the myosin-binding site is exposed by Ca2+-mediated, troponin-complex-dependent, tropomyosin movement; when Ca2+ is removed, the myosin-binding site is recovered by tropomyosin blocking;

troponin causes the location of tropomyosin to change, allowing the myosin-II to bind;

see diagrams

different types of muscle cells perform different functions

smooth muscle -

found in the walls of the stomach, intestine, uterus, arteries, and many other structures which undergo slow and sustained involuntary contractions; contraction is triggered by phosphorylation of myosin-II, or contraction is triggered by adrenaline, serotonin, prostaglandins, and several other signaling molecules;

cardiac muscle -

drives the circulation of blood; the heart contracts involuntarily for the life of the organism, up to 3 billion times during the average lifespan of a human; subtle abnormalities in actin or cardiac myosin-II can lead to serious disease; mutations in cardiac myosin-II genes in the sarcomere cause familial hypertrophic cardiomyopathy, a hereditary disorder responsible for sudden death in athletes

muscle contraction - essential concepts

(1) myosins are motor proteins that use the energy of ATP hydrolysis to move along actin filaments; in nonmuscle cells, myosin-I can carry organelles or vesicles along actin-filament tracks, and myosin-II can cause adjacent actin filaments to slide past each other in contractile bundles;

(2) in skeletal muscle cells, repeating arrays of overlapping filaments of actin and myosin-II form highly ordered myofibrils, which contract as these filaments slide past each other;

(3) muscle contraction is initiated by a sudden rise in cytosolic Ca2+, which delivers a signal to the myofibrils via Ca2+-binding proteins associated with the actin filaments

overview of the physical properties and functions of the 3 cytoskeletal systems in animal cells

!!!!memorize this table!!!!