Cytoskeleton: Components and Cell Movement

What are the major structural elements of the cytoskeleton?

Microtubules, microfilaments, and intermediate filaments.

The range of techniques used to understand the cytoskeleton include:

Fluorescence microscopy, digital video microscopy, and electron microscopy.

The cytoplasmic microtubules function to:

1) maintain axons, 2) maintain shape, 3) orient cellulose microfibrils, 4) mitotic and meiotic spindles for chromosome movements, 5) Vesicle movement.

The microtubule structure consists of:

Axonemal microtubules, the central shaft, alpha-tubulin and beta-tubulin molecules. Microtubule structure is made up of heterodimers of alpha tubular and beta tubulin (molecules are 4-5nm in diameter and 55 kDa, only share 40% amino acid sequence identity

Explain the structure of a microtubule

A hollow cylinder of 13 protofilaments around a lumen with an outer diameter (25 nm), inner diameter (15 nm) and dimer width of (8nm).

Singlet microtubules are

13 protofilaments

Doublet microtubules are

13 protofilaments plus 10 (or 11) - in cilica or flagella

Triplet microtubules are

13 protfilaments plus 10 (or 11), plus 10 - in basal bodies or centrioles

Microtubules form by an addition of:

tubulin dimers at the ends

Microfilaments

The smallest element of the cytoskeleton. Function in muscle contraction and cell migration. Monomers of G-actiin have a 7 nm diameter and polymerize into microfilaments of F-actin.

Roles of actin MFs in crawling cells

1) contractile bundle
2) gel (cell cortex)
3) parallel bundle (in filopodium)

Actin-binding proteins regulate actin:

Organization. Ordered actin arrays can be found in microvilli structures that are prominent in intestinal mucosal cells.

Actin MFs can be linked to membranes by proteins such as

spectrin and ankyrin

Networks of actin can be formed by

polymerization

Intermediate filaments

Are 8-12 nm in diameter and are tissue specific, coming in 6 classes. IFs are usually assemebled coils of homodimers into protofilaments, except keratin are made up of obligate heterodimers of type 1 and type 2 polypeptides.

Keratin

A well known protein essential for structures from animal skin

Two main systems used for cellular movement

1) specialized motor proteins & microtubules
2) actin microfilaments and myosin motor molecules

Kinesins and dyeins are:

motor proteins that couple ATP hydrolysis with the ability to walk along the microtubules

Kinesins move by

hydrolyzing ATP to move forward and releasing the nucleotide to release and then move. Most move towards the plus end of the microtubules.

The endomembrane system moves on

Microtubules and motor proteins

Dynein/dynactin complexes link to

cargo vesicles and move towards the minus end of microtubules.

Cilia and flagella are driven by

microtubules

Flagellum depend on

protofilaments and dynein for movement

Skeletal muscles are made up of

thick and thin filaments and are arranged into sarcomeres.

What are sarcomeres made up of?

Structural proteins and support proteins.

Nerve impulse travels

down the neuron

Action potential reaches the

neuromuscular junction: a synapsis between the neuron and muscle cell.

Depolarization of neuronal termini result in

release of acetylcholine which bind acetylcholine receptors on muscle cell surface

The four step cycle of muscle contraction

1) myosin binds loosely to actin filament
2) the power stroke, a trigger of the conformational change associated with move of thick versus thin filament
3) binding of ATP leads to change in myosin and weakening of bond to actin
4) ATP hydrolysis returns myosin to high energy state, ready for the next round of movement.

Actin microfilaments drive cell migration through cycles of:

1) protrusion
2) attachment
3) translocation
4) detachment