Tektins are heterodimeric polymers in flagellar microtubules with axial periodicities matching the tubulin lattice

Tektins are proteins that copartition with tubulin in a stable ribbon of three protofilaments from ciliary and flagellar microtubules. After purification, tektins A, B, and C from sea urchin sperm flagellar microtubules appear as extended relatively insoluble filaments, <5 nm in diameter. We used cross-linking reagents to investigate the associations and structural organization of subunits within tektin polymers isolated from stable protofilament ribbons of Strongylocentrotus purpuratus. We show by SDS-polyacrylamide gel electrophoresis, immunoblots, and transmission electron microscopy that tektins are continuous heteropolymers in the stable protofilament ribbons, and thus flagellar microtubules. Our results also provide evidence for the arrangement of different tektin polypeptides within 'core' filaments containing equimolar tektins A and B. Treatment of these core filaments with bis(sulfosuccinimidyl)suberate and with 1-ethyl-3-(3- dimethylaminopropyl)carbodiimide yielded a predominant cross-linked ~106- kDa heterodimer of tektins A and B; similar results were obtained by glutaraldehyde cross-linking of tektins solubilized under mild conditions. Finally, cross-linking with 3,3'-dithiobis(sulfosuccinimidylpropionate) revealed a 16 nm periodicity in isolated tektin AB filaments that can be related to the 8 nm tubulin dimer lattice and to periodically associated microtubule components.