Printed in Great Britain CONFIGURATION OF FLAGELLAR MICROTUBULE SUBUNITS

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 (a and /? tubulins) in an approximate 1:1 ratio on urea-polyacryl-amide gel electrophoresis. Heat (37 °C)-solubilized microtubules yield a protein fraction con-taining the tubulin dimer of molecular weight 115 000 Daltons. The dimeric tubulin subunit, as seen in the electron microscope, has an overall size of about 35 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 (35 ± 03 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 solubiliz-ing protofilaments shows the figure 8 (dimeric) subunits separating at random along the proto-filaments. 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 appear-ance of the protofilaments