Myosin Vis an abundant nonmuscle myosin that is responsible for the transport of cargo such as organelles in many cell types. Structurally, it consists of two identical polypeptide chains that dimerize to form a homodimer. The motor domains reside at theN-terminus of each chain and contain both ATP- and actin-binding sites. The motor domain is followed by a neck region containing six "IQ" motifs, each of which hinds calmodulin, a Ca²⁺-binding protein. The neck domain is followed by a region capable of forming coiled coils, via which the two chains dimerize. The final 400 amino acid residues form a globular tail domain (GTD), to which cargo binds. Myosin V would consume large amounts of ATP if its motor domain were always active, and a number of studies have been conducted to understand how this motor is regulated.

a. The rate of ATP hydrolysis (i.e., ATP molecules hydrolyzed per second per myosin V) was measured in the presence of increasing amounts of free Ca²⁺. The concentration of cytosolic free Ca²⁺ is normally less than 10^-6 M but can be elevated in localized areas of the cell and is often elevated in response to a signaling event. What do these data s uggest about myosin V regulation?

b. In additional studies, the ATPase activity of myosin V was measured in the presence of increasing amounts of F actin in the presence or absence of 10^-6 M free Ca²⁺. What additional information about myosin V regulation do these data provide?

c. Next, the behavior of truncate d myosin V, which lacks just its C-terminal globular tai l, was examined and compared to the behavior of intact myosin V. From this experiment, what can you d educe about the mechanism by which myosin V is regulated?

Transcribed Image Text:

12 10 8 10-6 [Ca2*1, M 10-4 Rate of ATP hydrolysis 16 12 +Ca2- -Ca2+ 10 15 [Actin], uM Rate of ATP hydrolysis