CB Exam 3 - Ch. 13, 14

Define cytoskeleton

a complex network of interconnected filaments and tubules that extends throughout the cytosol from the nucleus to the inner surface of the plasma membrane

Cytoskeleton provides an architectural framework for _________. It also plays an important role in ____-

cellular function; cell movement, cell division, organelle/vesicle, and protein/RNA movement

Cellular Movement involves

movement of a cell (or organism) through its environment

movement of components within the cell

shortening of the cell itself (contractility: the shortening of a muscle cell)

What are the components of Cytoskeleton?

microtubules, microfilaments, and intermediate filamentsMi

Microtubules

hollow tube with a wall consisting of typically 13 protofilaments; 25 nm in diameter

Microfilaments

two intertwined chains of F-actin; 7 nm in diameter

Intermediate Filaments

eight protofilaments joined end to end with staggered overlaps; 8-12 nm in diameter

What are the two types of Microtubules?

Axonemal microtubules and Cytosolic microtubulesA

Axonemal Microtubules

highly organized and stable

found in specific subcellular structure

associated with cellular movement (eg cilia, flagella, etc)

Cytosolic Microtubules

more loosely organized and dynamic network

located in cytosol

a variety of functions such as movement of chromosomes during meiosis and mitosis, movement of proteins/RNAs and vesicles/organells, and organization and maintenance of cell shape

Microtubule Structure

structure of a hollow cylinder enclosing a lumen

wall of cylinder consists of 13 protofilaments

polarity

entire diameter = 25 nm

inner opening diameter = 15

length = 8 nm

Tublin Heterodimers are

one alpha-tubulin + one beta-tubulin

building blocks of microtubules

Protofilament is a

linear polymer of tubulin heterodimers

All the tubulin heterodimers in the protofilaments have the

same orientation

Microtubule assembly requires

GTP and Mg 2+ for polymerization St

Steps of Microtubule Assembly

1. nucleation

2. elongation

3. protofilaments associate with each other side-by-side to form a sheet

4. sheets of 13+ protofilaments close into a tube, forming a microtubule

5. elongation continues by addition of dimmer at one or both ends

Nucleation

several dimers aggregating into clusters called oligomerEl

Elongation

oligomer grows by addition of subunits at either end, forming protofilament

There are ______ phases of microtubule assembly in vitro. The tubulin concentration at which MT assembly is exactly balanced by disassembly is called the ________.

3; critical concentration

MT Assembly - Lag Phase

MT formation is slow at first b/c the process of nucleation is slow

MT Assembly - Elongation Phase

much faster than lag phasef

MT Assembly - Plateau Phase

when the mass of MTs reaches a point where the amount of free tubulin is diminished, the assembly is balanced by disassembly

MT commonly originate from a structure in a cell called

microtubule-organizing center (MTOC)

MTOC serves as a site at which ___________ and acts as _______________.

microtubule assembly is initiated; an achor for one end of these microtubules

_________ of MTs are always _______________, and their ________________________________________.

Minus ends

anchored in the MTOC

plus ends always point outward toward the cell membrane

Define Centrosome

during interphase, many cells have an MTOC called the centrosome that is positioned near the nucleasC

Centrosome contains _________________.

two centrioles and associated pericentriolar material

Centriole walls are formed by ___________________ and may serve to _______________________.

nine sets of triplet microtubules; recruit pericontriolar material to the centrosome

Microtubules provide a rigid set of tracks for the _________________________.

transport of a variety of proteins/RNAs and organelles/vesicles

The mechanical work needed for movement depends on ____________________, which attach to proteins/RNA or vesicles/organelles and then “walk” along the MT

microtubule-associated motor proteins

_________ and ___________ are two major families of MT motors.

Kinesins and Dyneins

Centrosome is generally located near the ____________ and traffic can be _______ or _________.

center of the cell; inbound; outbound

Kinesins and Dyneins are motor proteins that couple

ATP hydrolysis to conformational changes to “walk” along MTs

Most kinesins move proteins/RNAs and vesicles/organelles

toward the plus end of MTs

Dyneins moves in the opposite direction,

toward the minus end of MTs

Kinesin contains 3 parts:

globular head, a coiled helical region, and a light-chain region

Globular Head is involved in ___________ and one of the two heads ______________.

hydrolzying ATP and attaching to MTs; moves forward to make an attachment to a new beta-tubulin subunit, followed by a detachment of the trailing head

Light-Chain Region is involved in attaching

the kinesin to cargo proteins/RNAs, vesicles, or organelles

What is a important function of kinesins?

Moving and localizing substances (proteins, RNAs, vesicles, organelles) within cells

Vesicles/organelles and proteins/RNAs toward MTOC are carried out by

dynein

Vesicles/organelles and proteins/RNAs away from MTOC are carried out by

kinesin

Microtubules function as a set of

tracks for the transport

Microfilaments (MFs) Function

best known for their role in the contractile fibrils of muscle cells

play important roles in cell movement, cytokinesis, and the development and maintenance of cell shape

MFs Function - Maintenance of Cell Shape Ex: Cortex

just beneath the plasma membrane, confers structural rigidity on cell surface and facilitates shape changes and cell movement

MFs structure

action is the protein building block

Action =

G-actin + F-actin

G-actin (globular actin)

free actins

F-actin (filamentous actin)

actins in MFs

MF is composed of two ________ wound around each other in a _______. The addition of each _________ is usually accompanied or followed by _____________.

linear strands; helix; G-actin; hydrolysis of the ATP molecule

MFs have _________: Plus end = ____; Minus end =

Which end grows faster?

polarity; end with ATP-actin; end with ADP-actin

plus end

Intermediate Filaments (IFs)

most stable and least soluble constituents of the cytoskeleton

do not have polarity

only found in multicellular organisms

IFs Function

play structure or tension bearing role

serve as a scaffold to support the entire cytoskeletal framework

Two IF-forming molecules =

dimer

Two dimers (IF) forms a

tetramer

IF - End to end alignment of tetramers form

protofilamentEi

IF Eight protofilaments assemble to form an

intermediate filament

MT resist _______ when a cell is ______

bending; compressed

MF serve as ______ elements that generate _______

contractile; tension

Intermediate filaments are ________ and can withstand ________.

elastic; tensile forces

Intermediate filaments are linked to both microtubules and microfilaments by proteins known as

spectraplakins (Plectin is one type of plakins)

Components of the cytoskeleton are

linked together both structurally and functionally