Moyes and Schulte Chapter 6 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin...

Moyes and Schulte Chapter 6

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Cellular Movement and Muscles


Cellular movement

Movement is a property of all cells

Some cells (such as this amoeba) can move through their environment

All cells can move components through the cytoplasm (such as the vesicles in this amoeba)


Cytoskeleton and Motor Proteins

All physiological processes depend on movement• Intracellular transport, changes in cell shape,

cell motility, and animal locomotion

All movement is due to the same machinery• Cytoskeleton – protein-based intracellular

network• Motor proteins – enzymes that use energy

from ATP


Cytokeleton

Composed of actin and microtubules

Fluorescently labeled cellActin – redMicrotubules – greenNuclei - blue


The Cytoskeleton and Movement

Three ways to use the cytoskeleton for movement• Cytoskeleton

roadway and motor protein carriers

• Reorganization of the cytoskeletal network

• Motor proteins pull on the cytoskeletal rope

Figure 6.1


Microtubules

Tube-like polymers of tubulin

Organized into many arrangements

Anchored near the nucleus and the plasma membrane• Microtubule-

organization center (MTOC) (-)

• Integral proteins (+)

Figure 6.2


Microtubule Structure

Figure 6.4

Polymers composed of the protein tubulin • Dimer of –tubulin and -

tubulin


Microtubules Composition and Formation

• Microtubules have a plus and minus end

Figure 6.5


Microtubules

• Minus end of the microtubule is anchored at the Microtubule-organization center (MTOC)

• Plus end of the microtubules anchored by Integral membrane proteins at the plasma membrane

Figure 6.2


Microtubules can grow and shrink

A microtubule can grow or shrink from either end“Dynamic Instability”

Fig 6.6a


Factors affecting Dynamic Instability

•Local concentration of tubulin affects microtubule dynamics

Figure 6.6


Microtubule dynamics regulated by MAPs

Figure 6.7

MAPs: Microtubule associate proteins

Bind to microtubulues and stabilize or destabilize structure


Motor proteins

• alpha-Tubulin: pale blue

• beta-Tubulin is pale green

• Kinesin walks towards the plus-end of microtubules (right side of picture)

Hoenger, A., Thormählen, M., Diaz-Avalos, R., Doerhoefer, M., Goldie, K.N., Müller, J. and Mandelkow, E. (2000) A new look at the microtubule binding patterns of dimeric kinesins. J Mol Biol, 297, 1087-103.

Motor proteins can move along microtubules


Movement Along Microtubules

Direction is determined by polarity and the type of motor protein• Kinesin move in + direction• Dynein moves in – direction

Fueled by ATP

Rate of movement is determined by the ATPase domain of the protein and regulatory proteins

Dynein is larger than kinesin and moves 5-times faster


Microtubule Functions

• Move subcellular components• e.g., Rapid change in skin color

Figure 6.3


Vesicle Traffic in a Neuron

Figure 6.8


Microtubule function - Cilia and Flagella

• Cilia – numerous, wavelike motion

• Flagella – single or in pairs, whiplike movement

• Composed of microtubules

• Arranged into axoneme

• Movement results from asymmetric activation of dynein

Figure 6.9


Microtubules and Physiology

Table 6.1


Microfilaments

Other type of cytoskeletal fiber

Polymers composed of the protein actin

Often use the motor protein myosin

Found in all eukaryotic cells

Movement arises from• Actin polymerization• Sliding filament model using myosin (more

common)


Actin filament Structure and Growth

• Polymers of G-actin called F-actin

• Spontaneous growth (6-10X faster at + end)

• Treadmilling when length is constant

• Capping proteins increase length by stabilizing minus end

Figure 6.10


Microfilament Arrangement

Figure 6.11


Actin Polymerization

Amoeboid movement

Two types• Filapodia are rodlike

extensions• Neural connections• Microvilli of digestive

epithelia

• Lamellapodia resemble pseudopodia

• Leukocytes• Macrophages

Figure 6.12


Myosin – a motor protein

Motor protein that works with actin filaments

Most common type of movement

Myosin is an ATPase • Converts energy from ATP to

mechanical energy

17 classes of myosin with

multiple isoforms

Similar structure• Head, tail, and neck

Figure 6.14


Myosin as a motor protein

Analogous to pulling yourself along a rope • Actin: the rope• Myosin: your arm

Myosin moves along actin


Sliding Filament model

Figure 6.15


Sliding Filament Model.

Two processes• Chemical

• Myosin binds to actin (Cross-bridge)

• Structural• Myosin bends

(Power stroke)

Cross-bridge cycle• Formation of cross-

bridge, power stroke, and release

Need ATP to attach and release

Figure 6.15


Variation in myosin function

Two factors• Unitary displacement

• Distance myosin steps during each cross-bridge cycle

• Depends on • Myosin neck length• Myosin placement

(helical structure of actin)

• Duty cycle• Cross-bridge

time/cross-bridge cycle time

• Typically 0.5• Use multiple myosin

dimers to maintain contact

Figure 6.16


Sliding filament assay

Figure 18-22, Lodish 4th edition. The sliding-filament assay


Actin and Myosin Function

Table 6.2

Muscle contraction

Moyes and Schulte Chapter 6 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin...

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