Cytoskeleton and Cell Junctions: Functions, Structures, and Regulation

What are the five primary functions of the cytoskeleton?

Cell adhesion, cell movement, intracellular transport, cell division, and maintaining cell shape.

What is the structural composition of intermediate filaments?

Overlapping tetramers that form rope-like, flexible, and non-polar filaments.

What is the function of the nuclear lamina?

To provide structural strength to the inner membrane of the nucleus.

How is the assembly and disassembly of nuclear lamins regulated during mitosis?

Disassembly occurs via phosphorylation by kinases, and reassembly occurs via dephosphorylation by phosphatases.

What are the primary functions of microtubules?

Cell movement (cilia/flagella), formation of the mitotic spindle, and intracellular transport.

What is the structural arrangement of a microtubule?

A hollow cylinder composed of 13 parallel protofilaments.

What are the structural differences between microtubules and intermediate filaments?

Microtubules are hollow, polar, and made of heterodimers; intermediate filaments are stacked, non-polar, and made of homodimers.

Which tubulin subunit forms the plus end and binds/hydrolyzes GTP?

Beta tubulin.

What is the significance of the 'GTP cap' in microtubules?

It stabilizes the plus end; its presence promotes growth, while its random loss leads to 'catastrophe' or shrinkage.

What is the role of Gamma tubulin ring complexes?

They serve as nucleation sites for microtubule growth within the centrosome.

In which direction does Kinesin transport cargo?

Toward the plus end (outward from the cell body to the terminal in neurons).

In which direction does Dynein transport cargo?

Toward the minus end (inward from the terminal to the cell body in neurons).

What are the primary functions of actin filaments?

Cell shape, motility, muscle contraction, microvilli formation, and contractile ring formation during division.

What is the structural composition of an actin filament?

Two protofilaments formed by stacking actin monomers, twisted into a helical shape.

How does ATP hydrolysis affect actin filaments?

It promotes depolymerization and decreases the stability of the filament.

What are the three phases of actin polymerization in a test tube?

Nucleation (lag phase), elongation (growth phase), and steady state (equilibrium phase).

What is 'treadmilling' in actin filaments?

A state where the rate of subunit addition at the plus end equals the rate of dissociation at the minus end.

How does Thymosin regulate actin polymerization?

It binds to actin subunits to prevent assembly, maintaining a pool of available monomers.

How does Profilin regulate actin polymerization?

It binds to actin and promotes assembly at the plus end.

What is the function of the Arp complex?

It nucleates actin filament growth by binding to the minus end of monomers to initiate branching.

What is the role of Myosin II?

It forms bipolar thick filaments that interact with unbranched actin to facilitate muscle contraction.

What are the three domains of a myosin motor protein?

The head (binds ATP/actin, ATPase activity), the neck (lever arm), and the tail (dimerization/aggregation).

Why are intermediate filaments considered stable?

They lack polarity and do not require ATP or GTP for assembly.

What determines the number of microtubules in a cell?

The number of available nucleation sites.

What determines the rate of microtubule growth?

The concentration of GTP-tubulin.

What is the difference between dynamic instability and treadmilling?

Dynamic instability involves rapid length changes at the plus end; treadmilling involves balanced addition and loss at opposite ends.

Which myosin type forms bipolar filaments in the sarcomere?

Myosin II

What is the function of formin?

It nucleates unbranched actin filaments by binding to the plus end.

What are the boundaries of a sarcomere?

Z-discs at the plus ends of actin filaments.

What protein caps the minus end of actin in a sarcomere?

Tropomodulin

What is the role of the troponin complex in muscle contraction?

It moves tropomyosin in response to calcium binding.

What triggers the release of calcium into the muscle cell cytosol?

Acetylcholine binding to its receptor.

What happens to actin and myosin filaments during muscle contraction?

They slide past one another without shortening.

What are the three main types of cytoskeletal filaments?

Intermediate filaments, microtubules, and actin filaments.

Which cytoskeletal filament is involved in forming the contractile ring during cytokinesis?

Actin filaments

What is the subunit of microtubules?

Tubulin heterodimers (alpha and beta).

How many protofilaments form a microtubule cylinder?

13

Which nucleotide is hydrolyzed by microtubules?

GTP

What is the primary function of epithelial tissues?

To bear mechanical stresses of tension and compression.

What is the basal lamina?

A specialized extracellular matrix on the basal side of epithelial tissues that provides a binding site for cell-matrix junctions.

What is the primary difference between epithelial and connective tissue regarding cell junctions?

Epithelial tissues have many cell-cell junctions, while connective tissues have very few.

Which junction type prevents molecules from leaking across epithelial sheets?

Tight junctions

What is the function of gap junctions?

To link the cytoplasm of adjacent cells for direct communication.

Which anchoring junction connects actin filaments to the extracellular matrix?

Actin-linked cell-matrix junctions (using integrins).

What is the difference between homophilic and heterophilic binding?

Homophilic binding involves the same protein type, while heterophilic binding involves different protein types.

What role does calcium play in cadherin function?

It acts as a cofactor for cell adhesion.

Which adaptor protein links cadherins to actin filaments in adherens junctions?

Catenin

What is the main structural difference between desmosomes and adherens junctions?

Desmosomes use intermediate filaments, while adherens junctions use actin filaments.

What are the transmembrane proteins that form gap junction channels?

Connexins

What component of the extracellular matrix enables it to withstand compression?

Glycosaminoglycans (GAGs)

What is the function of fibronectin in the extracellular matrix?

It organizes the matrix and helps cells attach to proteins.

Which transmembrane proteins mediate cell-matrix junctions?

Integrins