F-actin is not a straight polymer but can be considered a double-helical assembly. The two protofilaments twist around each other and cross over approximately every 13 actin subunits, and the subunits in each protofilament are repeated every 2.8 nm along the helix. F-actin is almost 8 nm wide. Different myosin motors have different "step lengths" on actin depending on their function. Which of the following step lengths would you expect to belong to a processive myosin motor that carries large cellular cargoes such as endocytic vesicles?
A) About 2.8 nm
B) About 8 nm
C) About 13 nm
D) About 27 nm
E) About 36 nm

Question

Quizplus · Accepted Answer

The protofilaments cross over approximately every 36 nm (i.e. 13 subunits × 2.8 nm/subunit), which means a processive myosin of this step size does not have to rotate around the filament as it moves along it. If the step size was not a multiple of the helical repeat, carrying large cargoes would involve a screw motion and be less efficient.

F-Actin Is Not a Straight Polymer but Can Be Considered