The ability of a population of fibroblasts to migrate along the surface of a tissue culture dish depends on adhesion between the cell surface and the extracellular matrix molecules coating the dish. The dish is coated with laminin, and the only cell-surface protein capable of binding laminin is a cell-adhesion protein called an integrin. Integrins are integral plasma-membrane proteins that function as heterodimers. Under these conditions the rate at which a fibroblast can migrate along the laminin coated culture dish is proportional to the strength of adhesion between the cell and the laminin substrate. The table below lists the rate of cell migration observed for fibroblasts genetically engineered to generate the indicated phenotypes. Microinjection into the cytoplasm of a wild. type cell of a solution of a synthetic peptide possessing the same sequence as the integrin beta subunit cytoplasmic domain would be expected to yield an average fibroblast-cell migration rat

Fibroblast Phenotype	Level of Integrin Heterodimer at the Cell Surface (percent of wild type)	Rate of Cell Migration (pm/min)
1. Wild type	100	2
2. Overexpression of the wild-type integrin alpha subunit	104	2
3. Overexpression of an integrin beta subunit lacking the cytoplasmic domain	96	0.6
4. Overexpression of the soluble cytoplasmic domain of an integrin beta subunit	98	0.6
5. Absence of the integrin alpha subunit	Less than 1	0.05

#Question id: 2577

#Unit 2. Cellular Organization

Which of the following occur(s) during prophase?

a. Kinetochores capture microtubule (+) ends.

b. Chromosomes move poleward.

c. Chromosomes align on the spindle equator.

d. Centrosomes separate and the spindle begins to form.

e. Chromosome condensation begins.

#Question id: 2578

#Unit 2. Cellular Organization

Following statements are regarding to proteins which involve actin filament assembly.

A. To nucleate the assembly of branched actin, Arp2/3 needs to be activated by interacting with a nucleation promoting factor (NPF), in addition to associating with the side of a pre-existing actin filament.

B. The NPF WASp is inactive due to an intramolecular interaction that masks the WCA domain.

C. Two NPFs each bind an actin subunit at their WH2 domains, and together, they activate the Arp2/3 complex through its interaction with their connector and acidic domains.

D. The NPF WASp is activated by a coincidence detection mechanism by which intramolecular interaction in WASp is relieved, allowing the W domain to bind actin and the acidic A domain to activate the Arp2/3 complex.

Which of the following statements represents regulation of the Arp2/3 complex by NPF?

#Question id: 2579

#Unit 2. Cellular Organization

Match the following proteins (Column I) with their functions (Column II) during cell organization and movement.

Which of the following is correct?

#Question id: 2580

#Unit 2. Cellular Organization

Following statements are regarding to mechanism of kinesin-1 movement.

A. Initially leading head tightly bound to the microtubule and not bound by any nucleotide, while the trailing head is weakly bound to the microtubule and has ADP bound.

B. Binding of ATP induces a conformational change causing the linker to swing forward and dock into the head. This motion swings the former trailing head to become the leading head.

C. Binding of ATP induces a conformational change causing the linker to swing forward and dock into the head. This motion swings the former leading head to become the trailing head.

D. When leading head releases ADP and coordinately the trailing head hydrolyzes ATP to ADP + Pi. Pi is released and the linker becomes undocked.

E. When trailing head releases ADP and coordinately the leading head hydrolyzes ATP to ADP + Pi. Pi is released and the linker becomes undocked.

Which of the following combination is correct?

#Question id: 2581

#Unit 2. Cellular Organization

Match the following toxins (Column I) with their effects (Column II) on actin.

#Question id: 2582

#Unit 2. Cellular Organization

Following statements are regarding to dynamics of actin filaments.

A. When the concentration of G-actin is above the Cc, the filament end will grow; when it is less than the Cc, the filament will shrink.

B. ATP–G-actin is added much faster at the (+) end than at the (−) end, resulting in a lower critical concentration at the (+) end than at the (−) end.

C. At steady state, actin subunits treadmill through a filament. ATP-actin is added at the (+) end, ATP is then hydrolyzed to ADP and Pi, Pi is lost, and ADP-actin dissociates from the (−) end.

D. ATP–G-actin is added much faster at the (-) end than at the (+) end, resulting in a lower critical concentration at the (-) end than at the (+) end.

E. At steady state, actin subunits treadmill through a filament. ATP-actin is added at the (-) end, ATP is then hydrolyzed to ADP and Pi, Pi is lost, and ADP-actin dissociates from the (+) end.

Which of the following statements are correct?