Chapter 9.1 - Cytoskeleton, cell cycle, and microtubules

different cytoskeletal components

• microtubules

• microfilaments

• intermediate filaments

general roles of cytoskeletal components

• structure and support

• intracellular transport

• contracility and motility

• spatial organization

cell polarity

spatial differences in shape, structure, and function within a cell

what types of cells exhibit polarity and why

almost all cell types exhibit some form of polarity; enables them to carry out specialized functions

largest of the cytoskeletal components of a cell

microtubules

types of microtubules

• cytoplasmic MT

• axonemal MT

cytoplasmic MT def

found in the cytosol and have various fnctions

cytoplasmic MT functions

• maintaing axons in nerve cells

• formation of mitotic and meiotic spindles

• placement and movement of vesicles

• maintaing or altering cell shape

axonemal MT def

include the organized and stable MTs found in structures specialized for movement

what structures might axonemal MTs be found

• cilia

• flagella

• basal bodies to which cilia and flagela attach

axoneme

central shaft of a cilium or flagellum, is highly ordered bundle of MTs

shape of MTs

straight hollow cylinders of varied length

what are MTs made of

longitudinal arrays of polymers called protofilaments

subunit of protofilament

heterodimer of tubulin, one alpha (a)-tubulin and one beta (b)-tubulin, these are globular proteins

how do the tubulin heterodimer subunits bind to each other

bind noncovalently

MAPs

microtubule associated proteins

what are MAPS composed of

a heterogenous collection of proteins with one domain that attaches to the side of a microtubule and another domain that projects outward as a tail

what do MAPs do

generally increase the stability of microtubules and promote their assembly

how is MAP activity controlled

by addition and removal of phosphate groups from amino acid residues

are the tubulin subunits ever found as individual monomers in the cell

no

what do both monomers of tubulin have

a GTP binding site

what can a-tubulin bind

GTP, it is physically trapped and NEVER hydrolyzed or exchanges

what can b-tubulin bind

either GDP or GTP (both hydrolyzable and exchangeable)

why do protofilaments have inherent polarity

• all the dimers in the MT are oriented in the same way

• orientaion of the dimers causes the protofilament to have polarity

t or f - the two ends of a protofilament are the same

false, the two ends differ both chemically and structurally

(-) end of the MT has

a-tubulin

(+) end of the MT has

b-tubulin

what does distribution of MTs help determine in the cell

shape of the cell

how are MTs arranged in cultured animal cells and how does that effect tje shape

extend in a radial array outwards from near the nucleus, gives cells a round, flattened shape

how are MTs arranged in columnal epithelial cells and how does that effect shape

oriented with their long axis parallel to the long axis to help support the cells elongated shape

what does treatment of cells with nocodazole or colchicine do

promotes MT disassembly and can disperse the Glogi elements into separate golgi stacks scattered throughout the cytoplasm

what happens when you remove colchicine or nocodazole from a cell where you had previously added it

the MTs reassemble and the Golgi membranes return to their normal position in the cell interior

kinesin

antegrade MT motor

dynein

retrograde MT motor

myosin

third motor proteins that carries organells along actin fibers

what is the energy source of motor protein movement

molecular motors convert energy from ATP into mechanical energy

how do motor proteins move

move unidirectionally along their cytoskeletal track in a stepwise manner and undergo a series of conformational changes that constitute a mechanical cycle

three categories of molecular mototrs

• move along microtubule tracks - kinesin and dynein

• move along microfilament tracks - myosin

how is the mechanical cycle of motor proteins fueled

coupled to the steps of a chemical cycle, which provide the energy necessary to fuel the motor’s activity to move along the track

what does the coupled chemical cycle of motor proteins include

• binding of ATP to the motor

• hydrolysis of ATP

• release of the products (ADP and Pi)

• binidng of new molecule of ATP

t or f - motor proteins are able to keep movigng a bit after energy input

false, motor proteins have virtually no momentum and are subjected to tremendous frictional resistance; will stop moving almost immediately once energy input has ceased

what direction does kinesin move in

plus end directed microtubule motor

how long is each step

approximately the lenght of one tubulin dimer

how much ATP does a single kinesin step require

one ATP

kinesin movement relationshop to ATP concentration

proportional

mechanism of kinesin movement

hand-over-hand mechanism

processivity of kinesin

highly processive, can walk along a MT for considerable distances without falling off

how does cytoplasmiv dynein move

like kinesin but in the opposite direction (negative end directed)

t or f - microtubules may bind kinesin adn dynein silmultaneously

true

t or f - organelles may bind both kinesin and dynein silmultaneously with one of them inactive

true

can movement along MTs be regulated

yes

melanosome aggregation is via what protein

dynein

melanosome dispersion is via what protein

kinesin

dynamic instabuluty

process of alternating between growing and shrinking

what is faster, dissociation of GDP or GTP tubulin dimer

GDP tubulin dimer dissociation is much faster than GTP tubulin dimer dissociation

function of a MT in a living cell is dependent on

its location and orientation

assembly of MTs in vitro from ab-tubulin dimers occurs in what two distinct phases

• a slow phase of nucleation in which a small portion of the MT is initially formed

• much more rapid phase of elongation

in vivo, how is the rate of nucleation of MTs different

much more rapid as it occurs in association with a variety of specialized structures calls MTOCs

role of MTOCs in all cells

control number of MTs, their polarity, the numebr of protofilaments that make up their walls, and the time and location of their assembly

centrosome

best studied MTOC

phases of the cell cylce

• M phase

• interphase

M phase include

mitosis and cytokinesis

interphase consitst of

G1, S, G2h

how long does mitosis last

about an hour

what consititutes most of the cell cylse

interphase, 90%, last longer than M phase

what happens in each stage of the cell cycle

5 stages of mitosis

prophase, prometaphase, metaphase, anaphase, telophase

what are centrosomes made of

complex of proteins not all of which have been identified

centrioles

short cylinder of modified MTs

gorwth of MTs occurs by addition of subunits to which end

plus end of the polymer away from the cnetrosome

minus end of MT is associated with what

the centrosome

nucleation of microtubule begins with

gamma-tubulun at the minus end

wat is nucelation initiated by

gammaTURCs - Tubuling Ring complex

the gamma-TURC is

a helical array of y-tubulin where ab=tubulin dimers assemble

thre classes of microtubules and where are they found

• all in mitotic spindle

• kinetochore

• astral

• polar

kinetochore MTs

connected to chromosomes (after first “finding” them via a kinetochore on the chromosome)

astral MT

project towards the cell cortex and interact with it thereby orienting the spindle of division

polar MTs

interact with microtubules from the opposite pole of the cell

what happens in prophase

• protein synthesis stops

• internal membrane systems that are normally associated with MTs disperse

• endocytosis and exocytosis stop

• Each centrosome (that divided in S phase) forms an MTOC and nucleation and elongation of microtubules begins

three major components of the nucelar envelope

• nuclear pores

• nucelar lamina (structural and composed of intermediate filaments)

• nucelar membranes

t or f - the three components of the nuclear envelope are dissassembled in the same process

false, in spearate processes

how is the nuclear membranes integrity first disupetd

mechanically as holes are torn into the envelope by cytoplasmic dynein molecules associated with outer nucelar membrane

how is the nucelar lamina disrupted

phosphorylation of the human nuclear lamin causes de-polymerization of the intermediate filaments in the nuclear lamina

what happens to the organelles during prophase

• mitochondria, lysosomes, and peroxisome remain relatively intact

• Golgi is either absorbed by the ER or fragmented and partitioned

what happens to the mitotic spindle in prometaphase

definitive mitotic spindle is formed and chromosomes are moved by microtubules into the center of the cell

kinetochore

a complex of proteins associated with the centromere of a chromosome during cell division to which the + end of microtubles of the spindle poles attach

polymerization vs depolymerization of MT

Plus end can add (polymerize) or lose (depolymerize) subunits

what si polymerizing and depolymerizing during prometaphase

Polymerization AND depolymerization at the plus end

Depolymerization at the - end

when is the cell in metaphase

when the fully condensed chromosomes are all aligned at the metaphase plate

metaphase plate

a plane equidistant between the two poles of the spindel

what happens in anaphase

• Tubulin subunits are lost from both ends of kinetochore microtubules

• Tubulin subunits are added to polar microtubules at the + end

• Tubulin subunits are lost from the minus ends of polar microtubules

• Note the role of motor proteins in pushing the polar microtubules apart

role of motor proteins in anaphase

• pushing polar MTs apart

• Pushing apart of the polar microtubules by a four headed kinesin family motor protein in image

final stage of mitosis

telophase

what happens in telophase

• mitotic spindle disassembles

• nuclear envelope of the two two nuclei are reassembled

• chromosomes become dispersd

what happens in cytokinesis

cytoplasm is partitioned into two cells

how does cytokinesis happen

• Cleavage depends on a belt-like bundle of actin microfilaments (the contractile ring) that form just below the plasma membrane in early anaphase

• As cleavage progresses, the ring tightens around the cytoplasm

what is important for completing cytokinesis

Signals emanating from the central part of the spindle

spindle midzone

central part of the spindle

contraction of actin ring during cytokinesis is generated by

interactions between actin and the motor protein, myosin

regulation through cell cycle is controlled by

cyclin dependent kinases (CDKs)