Chapter 17. Regulation of Myosin V
Introduction
Analyze the Data 17-1: Regulation of Myosin V
Myosin V is 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 the N-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 binds 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 suggest about myosin V regulation?
These data show that myosin V has low ATPase activity when the free Ca2+ is below 1 micromolar. Thus, one would expect that myosin V would be inactive except under conditions in which the cytosolic free Ca2+ were elevated above this value, perhaps in localized domains of the cell or upon excitation of excitable cells.
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