Cytosol and Cytoskeleton Part 2

What are microtubules composed of?

Tubulin, a heterodimer of alpha and beta tubulin

What nucleotide is required for microtubule polymerization?

GTP

What are the two ends of a microtubule called?

A growing (plus) end and a disassembling (minus) end

What is bound to the minus end of a microtubule?

A microtubule organizing center

What is present on the terminal dimer of the plus end of a growing microtubule?

GTP

What do microtubules interact with to regulate their function?

Microtubule-associated proteins (MAPs)

List the diverse cellular functions of microtubules

Transport and secretion of vesicular cargo, separation of chromosomes in mitosis and meiosis, structural stability in cilia and flagella, and motion

What is the diameter of a microtubule?

24 nm (with a 14 nm central lumen)

What structure does a microtubule's cross-section resemble?

A hollow tube made of 13 protofilaments arranged in a ring

What are the components of a centrosome?

A pair of centrioles surrounded by a centrosome matrix (pericentriolar material)

Describe the structure of a centriole

Composed of 9 triplets of microtubules arranged in a circle

How are the two centrioles of a centrosome oriented relative to each other?

Perpendicular to each other

What nucleates microtubules from the centrosome matrix?

γ-tubulin ring complexes at nucleating sites

What do centrosomes become during cell division?

The poles of the mitotic spindle

What is dynamic instability?

The constant contest between growth and degradation of microtubules

What favors continued growth at the microtubule plus end?

A GTP-bound plus end (GTP cap), where tubulin addition proceeds faster than GTP hydrolysis

What happens when a microtubule loses its GTP cap?

GDP-tubulin protofilaments peel away from the microtubule wall and depolymerize, releasing GDP-tubulin into the cytosol

Why does a GDP-bound plus end favor microtubule shrinkage?

Because it adds additional tubulin very slowly, favoring degradation

How does capping at the microtubule plus end by MAPs affect stability?

It prevents disassembly of the microtubule

What posttranslational modification enhances microtubule stability?

Enzymatic removal of the C-terminal tyrosine of the tubulin α subunit

Name high molecular weight MAPs that stabilize microtubules and assist in nucleation/bundling

High Mr MAPs and tau proteins

Name MAPs that favor microtubule disassembly

Stathmin and katanin

How is the direction of vesicle trafficking along microtubules determined?

By the location of the motor domain within the motor protein (N-terminal motor domain = plus-end directed; C-terminal motor domain = minus-end directed)

Which motor protein moves cargo toward the plus end of microtubules?

Kinesin

Which motor protein moves cargo toward the minus end of microtubules?

Cytoplasmic dynein

What structural domains do kinesin and dynein share?

A tail (cargo-binding), and globular heads (motor domains) that bind the microtubule

What links cargo to the tail of a motor protein?

An adaptor protein

Describe the ATP-dependent "walking" mechanism of motor proteins

The two globular heads alternately bind ATP, release phosphate, and exchange ADP for ATP, allowing the heads to alternately detach and reattach further along the microtubule, producing stepwise movement

In a neuron, what is outward transport (to the axon terminal) mediated by, and toward which end does it move?

Kinesin, moving toward the plus end

In a neuron, what is backward transport (to the cell body) mediated by, and toward which end does it move?

Cytoplasmic dynein, moving toward the minus end

What is the basal body of a cilium derived from?

A centriole

What is the microtubule arrangement within a cilium or flagellum called?

The axonemal 9+2 array

Describe the 9+2 microtubule arrangement

Nine outer doublet microtubules (A and B microtubule) surrounding a central pair of singlet microtubules

What structures connect the outer doublet microtubules to each other?

Nexin links

What structure connects the outer doublets to the central singlet microtubules?

Radial spokes

What generates the bending motion of cilia and flagella?

Dynein arms (outer and inner) that use ATP to walk along adjacent microtubule doublets

What happens when dynein produces sliding in isolated doublet microtubules versus an intact flagellum?

In isolated doublets, dynein causes the microtubules to slide past each other; in an intact flagellum, linking proteins between doublets convert this sliding into bending

What is required for coordinated ciliary beating?

Highly coordinated dynein activation/inhibition cycles on opposite sides of the axoneme

Give a physiological example of ciliary function

Cilia move mucus (with trapped inhaled particles) across the surface of the trachea and lung

What are intermediate filaments (IFs) composed of?

A heterogeneous group of fibrous proteins

How does the IF polymer form?

Spontaneously, in an energy-independent manner

Do intermediate filaments have polarity?

No, there is no polarity within the filaments

What are the primary functions of intermediate filaments?

Structural functions, such as linking Z-disks and forming neurofilaments

What are the four major classes of cytoplasmic/nuclear intermediate filaments?

Keratins (epithelia), vimentin and vimentin-related filaments (connective tissue, muscle, glial cells), neurofilaments (nerve cells), and nuclear lamins (all animal cells)

What are keratin filaments composed of, and where are they found?

Type I acidic keratins and Type II neutral/basic keratins (40-65 kDa), found in epithelial cells

What are neurofilaments composed of, and where are they found?

NF-L (70 kDa), NF-M (140 kDa), and NF-H (210 kDa) subunits, found in neurons

What are vimentin-containing filaments composed of, and where are they found?

Vimentin alone (fibroblasts), vimentin + glial fibrillary acidic protein (glial cells), or vimentin + desmin (muscle cells)

What makes up the nuclear lamina, and where is it found?

Lamins A, B, and C (65-75 kDa), found in all nucleated cells

Describe the basic assembly steps of an intermediate filament

Two monomers form a coiled-coil dimer, two dimers associate to form a tetramer (protofilament), and multiple protofilaments bundle together to form the mature filament

What is the axial repeat distance seen in assembled intermediate filaments?

21 nm

What happens to a sheet of epithelial cells linked by intermediate filaments (via desmosomes) when stretched?

The cells remain intact and together, distributing mechanical stress across the tissue

What junction connects intermediate filaments between adjacent epidermal cells?

Desmosomes

What junction connects intermediate filaments of epidermal cells to the basement membrane?

Hemidesmosomes