Actin and Intermediate Filaments (Lec. 6)

Question 1

Summarize the dynamics of actin filaments and the roles of actin-binding proteins

Answer 1

Actin monomers polymerize to form microfilaments with distinct + and - ends. The + end elongates by the addition of ATP-actin monomers. The growth of filaments is initiated by actin-binding proteins that can form either linear or branched filaments. Actin-binding proteins can also stabilize and organize filaments into bundles or networks.

Question 2

Illustrate the organization of actin filaments underlying the plasma membrane

Answer 2

Actin filaments are highly concentrated at the periphery of the cell. They determine cell shape and are involved in a variety of cell surface activities, including movement and anchoring to extracellular matrix.

Question 3

Describe the structure and function of microvilli

Answer 3

Microvilli are actin-based cell surface protrusions, extensions of the plasma membrane. They increase the exposed area available for absorption.

Question 4

Explain how remodeling of actin filaments is responsible for cell motility

Answer 4

Pseudopodia are extensions of moderate width, based on actin filaments, which are responsible for phagocytosis and for the movement of amoebas across a surface. Lamellipodia are broad, sheetlike extensions at the leading edge of fibroblasts, which contain a network of actin filaments. Filopodia are very thin projections of the plasma membrane, supported by actin bundles.

Question 5

Summarize the type of intermediate filament proteins

Answer 5

Type 1: acid keratins. Type 2: neutral or basic keratins. Type 3: vimentin, desmin, peripherin, glial fibrillary acidic protein. Type 4: neurofilament proteins, alpha-internexin, nestin. Type 5: nuclear lamins

Question 6

Describe the organization and function of intermediate filaments within cells

Answer 6

Extend from a ring surrounding the nucleus to the plasma membrane, where they’re anchored by desmosomes and hemidesmosomes. Provide mechanical support to nerve and muscle cells. Required for stability of cells in tissue but not for many cells in culture.

Question 7

Describe the structure and function of actin filaments (microfilaments)

Answer 7

Double-helix strands made of actin subunits (G-actin; G means globular); maintain cell shape by resisting tension, move cells via muscle contraction, divide animal cells in two, move organelles and cytoplasm in plants, animals, and fungi. Have a pointed end (-) and a barbed end (+). Bind ADP or ATP into a cleft (binding ATP promotes polymerization, and ATP eventually hydrolyzes).

Question 8

Describe the structure and function of intermediate filaments

Answer 8

Keratin, vimentin, lamin, or other fibers wound into thicker cables; maintain cell shape by resisting tension, anchor nucleus and some other organelles

Question 9

Describe the structure and function of microtubules

Answer 9

Alpha- and beta-tubulin dimers that form a hollow tube; maintain cell shape by resisting compression, move cells via flagella or cilia, move chromosomes during cell division, assist formation of cell plate during plant cell division, move organelles, provide tracks for intracellular transport

Question 10

What are the three general areas of cytoskeleton?

Answer 10

Cortical cytoplasm: just beneath the cell membrane; helps to maintain cell shape; dominated by an interconnected meshwork of microfilaments (actin); large membranous organelles and other bulky structures are mostly excluded from this region.
Subcortical cytoplasm or endoplasm: major organelles of the cell reside in this region and are spatially organized and often attached to cytoskeletal elements (all types of cytoskeletal proteins abundant here).
Nuclear cytoskeleton: important in mitosis; allows the nucleus to maintain and change its shape; nuclear lamins (intermediate filaments) give the nucleus form.

Question 11

What is nucleation in the formation of microfilaments?

Answer 11

the formation of trimers, followed by more active growth

Question 12

Describe actin polymers, and microfilament treadmilling.

Answer 12

Called F actin. - ends only add to + ends, creating polarity (this is reversible and depends on subunit availability). The + end grows 5-10 times faster than the - end. G actin bound to ATP favorably associates with the + end, and the binding of G actin will eventually lead to the hydrolysis of ATP to ADP. The presence of ADP on an exposed end promotes depolymerization.

Question 13

What does profilin do?

Answer 13

stimulates ADP exchange for ATP

Question 14

What does formin do?

Answer 14

catalyzes extension and nucleation of a microfilament

Question 15

What does Arp 2/3 complex do?

Answer 15

initiates the formation of branches

Question 16

What do capping proteins and tropomyosin do?

Answer 16

stabilize filaments

Question 17

What do cross-linking proteins do?

Answer 17

cross-link filaments into bundles and networks

Question 18

What does cofilin do?

Answer 18

severs actin filaments

Question 19

Do mature RBC’s have microtubules or intermediate filaments?

Answer 19

No

Question 20

What does spectrin do?

Answer 20

Important for stability and structure of cell membrane

Question 21

What does ankyrin do?

Question 22

What does protein 4.1 do?

Answer 22

maintains the shape of red blood cells

Question 23

What are focal adhesions?

Answer 23

Areas of the cell that make connections with the extracellular matrix (tend to have a lot of actin attachments, called stress fibers)

Question 24

What are stereocilia?

Answer 24

fingerlike extensions; not involved in absorption, but involved with detecting extracellular changes (like auditory hair cells); formed by actin bundles

Question 25

What are the proposed steps in locomotion?

Answer 25

1. Protrusion: extensions probe forward in search of a new substratum contact site. 2. Substrate adhesion: forward extension is stabilized if it finds an adhesive region, and the mechanism of attachment involves integral membrane receptors and their coupling to internal cytoskeleton elements. 3. Traction/Cell Body Movement: the bulk of the cell's cytoplasm moves forward

Question 26

What do Rho family proteins do?

Answer 26

They're coupled to receptors, and respond to environmental cues by activating actin binding proteins, which stimulates actin remodeling and cell movement.

Question 27

What do intermediate filaments do?

Answer 27

Important for cell scaffolding and structural integrity.

Question 28

Describe the structure and assembly of intermediate filament proteins.

Answer 28

1) two polypeptides dimerize, connected by the central rod domain (coiled-coil); this process is in parallel (N-N/C-C) 2) two dimers combine to form a tetramer; this process is in antiparallel (N-C/C-N) 3) tetramers stack on one another end to end to form a protofilament 4) 8 protofilaments combine, twisting around each other to form a rope-like structure called a filament

Question 29

Describe the functions of intermediate filaments

Answer 29

1) Stable structure and support within the cell (of the plasma membrane and nuclear membrane) 2) Transmitting force between cells or to the extracellular matrix (resisting and distributing externally generated stresses over multiple cells to prevent damage to individual cells) (desmosomes are junctions between cells, and hemidesmosomes are junctions between cells and connective matrix)