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Lecture 16 Motor proteins and cytoskeletal-mediated cell behavior

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Lecture 16 Motor proteins and cytoskeletal-mediated cell behavior. Myosin II and myosin II bipolar thick filament. 2 heavy chains: Globular head domain (ATP) long tails (coiled coils) 2 light chains:. Bipolar thick filaments: Tail-tail interactions. Hundreds of myosin II molecules.

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Lecture 16 Motor proteins and

cytoskeletal-mediated cell behavior

Myosin II and myosin II bipolar thick filament

2 heavy chains:

Globular head domain (ATP)

long tails (coiled coils)

2 light chains:

Bipolar thick filaments:

Tail-tail interactions

Hundreds of myosin II


Thin filament: actin filament

Experimental evidence for the motor activity of the myosin head

0.6 sec apart

4 um/sec

Myosin head S1:

by chymotrypsin and papain

Attached to the glass slide

Myosin superfamily

Similar motor domains

Move to plus end except VI

Small insertion in the motor domain

Function of myosins

Myosin II: contractile activity in muscle and nonmuscle cells, cytokinesis

(pinching apart of a dividing cell into two daughters), forward translocation of

cell body during migration);

Myosin I: contain a second actin-binding or membrane-binding site in their tails-

Intracellular organization and the protrusion of actin-rich structures at the cell surface;

Myosin V: vesicle and organelle transport;

Myosin VII: inner ear (deafness when mutated)

Microtubule motors


Dyneins: minus-end-directed

Unrelated to the kinesin superfamily

Cytoplasmic dyneins: vesicle trafficking and Golgi apparatus localization;

Axonemal dyneins: sliding movements of MT(14um/sec)

Third: cilia beating

Kinesin-related proteins, KRPs

Most of them have N-terminal motor domains walk toward plus end

One family has C-terminal motor domains and move to minus end

BimC forms bipolar motor

Cycles if structural changes in myosin

Head-over-head movement of kinesin

5 nM

Attachment of dynein to a

membrane-enclosed organelle

Organization of Golgi by MT

Large complex!

Colchicine or nocodazole

causes ER to collapse to center

and Golgi to fragment and disperse

Kinesin: motor protein receptors

Myofibrils under EM

Skeletal muscle cells

(muscle fibers)



Myofibrils: 1-2 um diameter,

made of sarcomeres, 2.2 um long

Z-disc:plus end

Sliding-filament model of

muscle contraction

300 heads

Shorten by 10% in

<1/50th sec

15 um/sec

Titin: “molecular spring”

Stable actin


“Molecular ruler”

Nebulin: repeating 35 aa

actin-binding motif

The control of skeletal muscle contraction by troponin

Troponin I-T complex

pulls the the tropomyosin

out of its normal into a

position along the actin

filament that blocks the

the binding of myosin heads

Upon Ca++ increase,

troponin C causes

troponin I to release its

hold on actin, allowing

tropomyosin to slip back so that

myosin head can walk along

the actin filament

Effect of the heart of a suble mutation in the cardiac myosin


Inherited 2/1000: heart enlargement,

abnormal small coronary vessels,

disturbances of heart rhythm (cardiac

arryhthmias)--mutations in myosins and

Contractile proteins.

Dilated cardiomyopathy:

cardiac actin mutations--early heart failure

Familial hypertrophic cardiomyopathy

Frequent cause of sudden death

in young athletes

The arrangement of MTs

in a flagella or cilium

Contrasting motions of

flagella and cilia

MT and dynein based motility structures

“9+2” arrangement

In sperm and


Whip-like motion

of cilia

Move cells

or liquid or

other cells

Base binds to A MT

and heads bind to B MT

The bending of an axoneme

The motor action causes

a bending motion, creating

waves of beating motion as

in a sperm.

Bacterial flagella don’t have

MT or dynein and do not wave

Or beat. Instead, long, rigid, helical

Filaments of repeating subunits of flagellin.

Move like propellers driven by rotary motor

in the cell wall. The name is a mistake…

Basal bodies

Kartagener’s Syndrome:

Male sterility (immotile sperm)

Higher susceptibilty of long infections

Left-right body axis defects

How basic cytoskeletal mechanisms produce complex cell behaviors

Cell shape



Polarity of actin patches and cables throuhgout the yeast cell cycle

New patches

Very few cytoplasmic MT

Cables random

Actin patches are concentrated at the growing tip of the bud

Actin cables align and point toward them

Morphological polarization of yeast cells in response to mating factor

Yeast cells can’t swim

a-cell and a-cell, two mating types

Shmoo tip grow toward the highest concentration of the signal molecule

Behavior of lamellipodial fragments

Lamellipodia contain all of the machinery

that is required for cell motility!

Lamellipodia and ruffles at the leading edge

Lamellipodia that fail

to attach to the

substratum are swept

backward-- ruffling

Neutrophil polarization and chemotaxis

Peptide formyl Met-Leu-Phe

New lamellipodium toward the tip

Extends lamellipodium polarizes

cytoskeleton and cell moves forward

The complex morphological specialization of nerrons depends

on the cytoskeleton

Anterograde transport

Retrograde transport

Cytoskeleton provide the engine for construction

of the entire nervous system

As well as the supporting structures that strengthen, stabilize

and maintain its parts


Actin-based motors, microtubule motors,

their structure and function

Motility machines: myofibrils and flagella and cilia

Complex cellular behaviors mediated by cytoskeleton

Cell polarization: yeast budding and mating

Cell crawling