Chapter 9 - The Cytoskeleton and Cell Motility

What does the cytoskeleton provide?

structure, organization, motility

Functions of the cytoskeleton? (SOTFC)

• maintains shape

• positions organelles

• intracellular transport

• force generation for movement

• cell division

What are the three components of the cytoskeleton?

1. microtubules

2. actin filaments (microfilaments)

3. intermediate filaments

Kinesin structure

tetramer (2 heavy + 2 light chains) with motor heads using ATP

What end of the microtubule does kinesin move towards?

the plus end

Kinesin function

transports organelles and vesicles outward

Cytoplasmic dynein structure

Huge complex; 2 heavy + intermediate + light chains

What end of the microtubule does cytoplasmic dynein move towards?

the minus end

The movement of cytoplasmic dynein is considered

retrograde movement

The movement of kinesin is considered

anterograde movement

Cytoplasmic Dynein function

works with kinesin in opposite directions on same MTs

MTOCs stands for

Microtubule-Organizing Centers

MTOCs function

nucleate and anchor MTs,

control: number, polarity, and assembly

Centrosome structure

2 centrioles + pericentriolar material

Each centriole =

9 triplet mictrotubules

growth for MTOCs occur at

plus end, away from centrosome

Basal bodies structure

same structure as centrioles; nucleate cilia and flagella

γ-Tubulin Ring Complex (γ-TuRC)

Template for MT nucleation

γ-TuRC is found in

PCM

Three main types of molecular motors?

Kinesin, dynein and myosin

What bonds hold cytoskeletal polymers together?

weak, non-covalent interactions

What is the basic building block of a microtubule?

alpha and beta-tubulin dimers arranged into protofilaments

Which end of a microtubule has the beta-tubulin and which has the alpha-tubulin?

beta: plus

alpha: minus

Name two drugs that cause microtubule disassembly 

Colchicine and nocodazole

What happens to the Golgi if microtubules disassemble?

It desperses

When does the Gogli re-form?

When MTs reassemble

What energy source drives motor proteins?

ATP hydrolysis

Which motors run on microtubules?

kinesin and dynein

Which motor runs on actin?

myosin

How far does kinesin move per ATP?

8 nm (one tubulin dimer)

What do MTOCs control?

Microtubule number, polarity, location, and timing of assembly

What is the main MTOC in animal cells?

the centrosome

What protein intitates microtubule nucleation?

gamme-tubulin in the gamme-TuRC

Oritentation of microtubules in the cytoplasm?

Minus ends anchored at centrosome; plus ends extend outward

What is “dynamic instability”?

Alternating growth and shrinkage of a microtubule end

When does a microtubule grow?

When a GTP-tubulin cap is present

What happens if the GTP cap is lost?

Catastrophe - rapid depolymerization

What does GTP —> GDP hydrolysis do to tubulin?

Causes it to bend and destablize the filament

What is the core of a cilium or flagellum called?

the axoneme

Describe teh axoneme arrangment

9 + 2 array - nine doublet MTs arround two singlets

Nexin links

elastic brdiges limiting doublet sliding; convert sliding into bending

What powers ciliary motion?

Dynein arms on A-tubules sliding against B-tubules

What are the two strokes of ciliary beating?

1. Power stroke (rigid push)

2. Recovery stroke (flexible return)

What distinguishes intermediate filaments from other cytoskeletal elements?

• only in animal cells

• no ATP/GTP

• no polarity

• chemically heterogenous

Describe IF assembly

two monomers —> dimer —> tetramer —> 8 tetramers —> filament

Main function of IFs

provide mechanical strength and resilience

What happens if keratin IFs are absent?

severe skin blistering from minor trauma

What happens if desmin IFs are absent?

misaligned myofibrils and muscle weakness

Actin filmaent diameter

8nm

What nucleotide is required for actin polymerization?

ATP

Which end of actin grows faster, and by how much?

The barbed (+) end; ~10x

What is “treadmilling”?

balanced addition at + end and loss at - end —> constant length

What determines growth rate at each end of actin?

Critical concentration of ATP-actin monomers

General direction of myosin movement?

towards barbed (+) end (except myosin VI)

What are the two classes of myosin?

1. Conventional (Type II)

2. unconventional

Funciton of myosin II

Drives muscle contration and non-muscle motility (cell division, migration)

Structure of myosin II

2 heavy + 4 light chains; bipolar filament with heads at ends

Funciton of myosin V

Hand-over-hand motor (~36nm steps) for vesicle and organelle transport

How are vesicles transported overall?

long distance on: microtubules (kinesin/dynein)

local on: actin (myosin)

Sarcomere

contractile unit between two Z lines in a myofibril

What filaments are in each band?

I band: actin

A band: overlap

H zone: myosin

M line: center

What happens during muscle contraction?

Actin and myosin slide past each other

A band: constant

I and H: shorten

What produces force in the sliding-filament model?

Cross-bridge cycling of myosin heads on actin

What drives crawling movement in cells?

Actin polymerization at the lamellipodium and myosin contraction at the rear

Steps of cell crawling? (PACD)

1. protrusion of lamellipodium

2. adhesion via integrins

3. contraction to pull cell body forward

4. rear detachment

What are integrins’ roles in motility?

Mediate temporary attachment to substratum

Lamellipodia

broad, flat actin-rich extensions at the leading edge

How do myosins aid motility?

Generate contractile forces to retract the rear of the cell