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Why do actin filaments have a distinct polarity?

The asymmetrical actin monomers associate in a head-to-tail fashion to form actin filaments. Since all the monomers are oriented in the same direction, the filament has a distinct polarity.

Why is the polarity of actin filaments important in muscle contraction?

The polarity of actin filaments defines the direction of myosin movement. If actin filaments were not polar, the unidirectional movement of myosin that results in the sliding of actin and myosin filaments could not take place.

What is treadmilling, and at what concentration of monomers does it occur?

Treadmilling is a dynamic behavior of actin filaments (or microtubules) in which they maintain a near-constant length by addingt ATP-actins (or GTP-tubulins) to the barbed (or plus) end and dissociating an equal number of ADP-actins (or GDP-tubulins) from the pointed (or minus) end. During this steady-state behavior, subunites hydrolyzed their nucleoside triphosphates after assembly, flux throught the filament, and exit from the pointed end. Treadmilling occurs at a monomer concentration between the critical concentation for the barbed end and the critical concentration for the pointed end.

How would cytochalasin and phaloidin affect treadmilling actin filaments?

Cytochalain binds to the barbed ends of actin filaments and blocks their elongations, so it would lead to depolymerization of treadmilling filaments. Phalloiding binds to actin filaments and inhibits their depolymerization, so it would cause filaments to stop treatmilling but remain present and grow longer.

How do ADF/cofilin, profilin, and the Arp2/3 complex regulate actin filament assembly and turnover?

ADF/cofilin binds to actin filaments and increases the dissocation of actin monomers. Profilin binds actin/ADP and stimulates the exchange of ADP for ATP, forming actin/ATP monomers that can join growing filaments. the Arp2/3 complex initiates the formation of branches.

Which bands or zones of a muscle sarcomere change length during contraction?

The I band and the H zone shorten during contraction.

Why doesn't the A band change length?

The A band doesn't shorten because it is occupied by thick myosin filaments.

How does Ca2+ regulate the contraction of smooth muscle cells?

The contraction of smooth muscle cells is regulated by the phosphorylation of the myosin regulatory light chain by myosin light-chain kinase, which in turn is regulated by association with the calcium-binding protein calmodulin. An increase in the cytosolic concentration of calcium leads in the binding of calmodulin to myosin light-chain kinase.

How would expression of siRNAs targeted against vimentin affect the growth of fibroblasts in culture?

Intermediated filaments are not required for the growth of cells in culture, so siRNA against vimentin would have no effect.

Why are intermediate filaments apolar,even though they are assembled from monomers that have distinct ends?

Dimers of cytoskeletal intermediate filaments assemble in a staggered antiparallel manner to form tetramers, which can then assemble end-to-end to form protofilaments. Since they are assembled from antiparallel tetramers, intermediate filaments do not have distinct ends and are apolar.

What key observation helped Vale and colleagues devise a strategy for the isolation of kinesin?

The in vitro movement of microtubules required ATP and was inhibited by the nonhydrolyzable ATP analog aMP-PNP. Importantly, organelles remained attached to the microtubules in the presences of AMP-PNP, suggesting that the motor proteins responsible for organelle movement might also remain attached.

How would the removal of nexin affect the beating of cilia?

By the linking of the microtubule doublets in cilia together, nexin converts the sliding of individual microtubules to a bending motion that leads to the beating of cilia. If it were eliminated, the microtubules would simply slide past one another.

You are studying transport of secreatory vesicles containing insulin along microtubules in cultured pancreatic cells. How would treatment with colcemid affect the transport of these vesicles?

Colcemid inhibits the polymerization of microtubules and would inhibit the transport of secretory vesicles along microtubules.

What is the cellular function of gamma-tubulin?

Gamma tubulin plays a key role in the formation of microtubule organizing centers. Gamma tubulin associates with other proteins to form the gamma tubulin ring complex, which functions as a seed for te nucleation of new microtubules.

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