Journal of Cell Science, Vol 15, 495-511, Copyright © 1974 by Company of Biologists

Submitted on January 21, 1974

Configuration of Flagellar Microtubule Subunits

¹ Department of Biology, Biological Research Laboratories, Syracuse University, Syracuse, New York 13210, U.S.A., and Department of Cell Biology, Centro de Investigation del I.P.N., Mexico 14, D.F.

Microtubule protofilaments and their subunits isolated from sperm flagellar doublet tubules of the sea urchin Strongylocentrotus purpuratus were examined by analytical biochemistry and high-resolution negative staining electron microscopy. All microtubule (tubulin) fractions show 2 polypeptide bands ( and tubulins) in an approximate 1:1 ratio on urea-polyacrylamide gel electrophoresis. Heat (37 °C)-solubilized microtubules yield a protein fraction containing the tubulin dimer of molecular weight 115000 Daltons. The dimeric tubulin subunit, as seen in the electron microscope, has an overall size of about 3.5 x 8 nm and appears to have the configuration of a figure 8 because of stain penetration into the centre of each of its 2 halves (figure os). Isolated protofilaments (3.5 ± 0.3 nm thick) can each be resolved into 2 subfilaments (1.7 ± 0.2 nm thick). The 2 subfilaments have periodic lateral associations resulting in the basic 4-nm subunit repeat (figure o) along the protofilament. Examination of collapsed and solubilizing protofilaments shows the figure 8 (dimeric) subunits separating at random along the protofilaments. We conclude that the tubulin dimer must be composed of either elongated or bilobed monomers which result in the figure 8 configuration and hence the 2-stranded appearance of the protofilaments.