The structure of 14-protofilament microtubules reassembled from dogfish shark brain tubulin was analyzed by high resolution electron microscopy and optical diffraction. The simultaneous imaging of the protofilaments from near and far sides of these tubules produces a moire pattern with a period of ~96 nm. Optical diffraction patterns show that the 5-nm spots that arise from the protofilaments for the two sides of the tubule are not coincident but lie off the equator by a distance of 1/192 nm-1. These data provide evidence that in reassembled microtubules containing 14 protofilaments, the protofilaments are tilted 1.5° with respect to the long axis of the tubule, giving a lefthanded superhelix with a pitch of 2.7 µm. The hypothesis is that the tilt of the protofilaments occurs to accommodate the 14th protofilament. It is determined that when the 14th protofilament is incorporated, the 3-start helix is maintained, but the pitch angle changes from 10.5° to 11.2°, the angle between protofilaments measured from the center of the microtubule changes by 2°, and the dimer lattice is discontinuous. These observations show that the tubulin molecule is sufficiently flexible to accommodate slight distortions at the lateral bonding sites and that the lateral bonding regions of the a and β monomers are sufficiently similar to allow either α-α and β-β subunit pairing or α-β subunit pairing.
ASJC Scopus subject areas
- Cell Biology