A Microtubule Appears as a Left-Handed Helix Due to an Approximately

Multiple Choice

A microtubule appears as a left-handed helix due to an approximately 0.9-nm stagger in the lateral contacts between adjacent protofilaments. In the lateral contacts, α- and β-tubulins in one protofilament interact with α- and β-tubulins, respectively, in the neighboring protofilament, except for a longitudinal discontinuity along the microtubule called the seam. Along the seam, lateral contacts have to be made between different tubulins (i.e. α-tubulin with β-tubulin) . Which of the following do you think is acceptable as the repeat distance of tubulin monomers along a protofilament?

A) 4 nm
B) 5 nm
C) 6 nm
D) 7 nm
E) 10 nm