Recent structural studies indicate that a tektin heteropolymer forms a unique protofilament of flagellar microtubules. We report here the sequence of tektin C (~ 47 kDa), predicted from its cDNA (GenBank U38523), compared to tektins A (~ 53 kDa) and B (~ 51 kDa) from sea urchin (Strongylocentrotus purpuratus) sperm flagellar microtubules, and compared to partial sequences reported from mouse and human. We are now able to make several observations concerning the tektin family: (1) their common structural features, (2) a comparison of their structure to intermediate filament proteins, and (3) their possible organization in the tektin filament polymer. The predicted amino acid sequence identities/similarities are: for tektins A and C, 42/54%; for tektins A and B, 34/51%; for tektins B and C, 29/42%; for tektin C and a partial cDNA clone from mouse testis, 55/65%; and for tektin B and a partial cDNA clone from human brain, 45/47%. The three tektins (and the human clone) possess the exact sequence repeat RPNVELCRD. The structural pattern of all three tektin polypeptides is similar to intermediate filament proteins. Tektins are predicted to form extended rods composed of two α-helical segments (~ 180 residues long) capable of forming coiled coils, which are interrupted by short non-helical linkers. The two segments are homologous in sequence and secondary structure, indicating a gene duplication event prior to the divergence of the three tektins. Along each tektin rod cysteine residues occur with a periodicity of approximately 8 nm, coincident with the axial repeat of tubulin dimers in microtubules. From EM data and calculations of secondary structure, the segment length of tektin AB heterodimers is likely to be 16 nm. Both segments of tektin C may be 24 nm long, but one may be 16 nm. On the basis of the available evidence, we propose that coassembly of tektin AB heterodimers with tektin C dimers produces filaments with overall repeats of 8, 16, 24, 32, 40, 48 and 96 nm, generating the basis for the complex spatial arrangements of axonemal components.
Bibliographical noteFunding Information:
This work was supported by US Public Health Service grant GM35648, Minnesota Medical Foundation grants SMF-527-86 and FSW-155-95 to R.W.L., by National Research Service Award 1F32-GM11669 to J.M.N., and by National Science Foundation Research Training Group grant BIR 9113444. We thank the reviewers for suggestions to improve the original manuscript.
- Coiled coil
- Intermediate filament