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

Question

Quizplus · Accepted Answer

Thirteen protofilaments have a total of thirteen staggers between them. Thus, a full turn of the helix has a rise of approximately 12 nm (= 13 staggers × 0.9 nm/stagger). Since there is a seam, this rise should correspond to an odd number of monomer distances, i.e. 1, 3, 5, etc. In reality, the number is 3, equivalent to an axial tubulin displacement of 4 nm (= 12 nm/3).

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