INTRODUCTION Tektins were originally identified as a set of proteins com- prising relatively insoluble, 2-3 nm diameter filaments that remain after the extraction of sea urchin sperm flagellar microtubules with chaotropic solvents (Linck and Langevin, 1982). Three tektins have so far been characterized: tektin A (~53 kDa), B (~51 kDa) and C (~47 kDa). Tryptic pep- tide mapping (Linck and Stephens, 1987) and immunolog- ical studies (Linck et al., 1985; Steffen and Linck, 1988) demonstrated that the three tektins were different but related polypeptides and that they were unrelated to tubulin. Initial biochemical and structural data suggested that these novel proteins were similar to intermediate filament protein (IFP) from vertebrates in terms of their relative insolubility, mol- ecular masses, and fibrous, α-helical substructure (see Linck, 1990). Immunological studies further substantiated the similarity between tektins and IFP (Chang and Piperno, 1987; Steffen and Linck, 1989a). It was surprising, there- fore, that the first sequence reported for tektin A (Norran- der et al., 1992) revealed only a weak homology with IFP. The organization of tektins in microtubules is not known in detail, but tektins probably interact directly with tubulin. Immunofluorescence microscopy has demonstrated that tek- tins A, B and C are present in all nine flagellar doublet microtubules and possibly in the central pair microtubules (Steffen and Linck, 1988). Immunoelectron microscopy suggests that tektins form extended polymers in the walls of ciliary and flagellar microtubules (Linck et al., 1985). Sarkosyl detergent extraction of cilia and flagella yields insoluble ribbons of approximately three protofilaments, composed of tubulin and tektins in a close molar ratio, sug- gesting a direct molecular interaction (Linck, 1976). Finally, at least one form of the insoluble protofilament rib- bons is localized to that part of the A-tubule to which the B-tubule and nexin links join (Linck, 1976; Stephens et al., 1989). Immunological evidence suggests that tektins or tektin- related polypeptides are present in other microtubule sys- tems. Besides staining flagellar axonemes, tektin antibod- ies also stain basal bodies in sea urchin sperm (Steffen and Linck, 1988). This result is not unexpected, since the dou- blet tubules assemble from the plus ends of the basal body triplet microtubules (Alberts et al., 1989; Allen and Borisy, 1974), and therefore, some proteins might be common to both doublet and triplet microtubules. Perhaps more sur- prisingly, tektin antibodies also stain centrioles in a variety of mammalian cells, including HeLa cells (Steffen and Linck, 1988), suggesting that tektins could be evolutionar- ily conserved components of centrioles. Tektin-like com- ponents have also been detected in centrosomes and in spin- dles and midbodies of dividing cells (Steffen and Linck, 1989b, 1990, 1992, and unpublished observations). We report here the cloning and sequencing of a cDNA 909 Journal of Cell Science 106, 909-918 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 Tektins are a class of proteins that form filamentous polymers in the walls of ciliary and flagellar micro- tubules, and they may also be present in centrioles, cen- trosomes and mitotic spindles. We report here the cloning and sequencing of a cDNA for ciliary tektin B1. Comparison of the predicted amino acid sequence of tektin B1 with the previously published sequence for tektin A1 reveals several features that better define this class of proteins. Like tektin A1, the central region of the tektin B1 polypeptide chain is predicted to form a coiled-coil rod, consisting of four major -helical regions that are separated by non-helical linkers. Between the central rod domains of tektins A and B there is a 34%/20% amino acid sequence identity/simi- larity, including equivalent 50-residue segments con- taining 36 identities, and a high probability of long- range structural homology. The tektin polypeptide chains are divided into two major segments that have significant sequence homology to each other, both within a given tektin chain and between tektins A and B, indicative of gene duplication events. The tektins have a secondary structure and molecular design similar to, but a low primary sequence homology with, intermedi- ate filament proteins. Unlike tektin A1, tektin B1 lacks any part of the C-terminal IFP consensus sequence. Key words: centrioles, cilia, coiled-coil proteins, flagella, intermediate filaments SUMMARY Tektin B1 from ciliary microtubules: primary structure as deduced from the cDNA sequence and comparison with tektin A1 Runtian Chen 1 , Catherine A. Perrone 1 , Linda A. Amos 2 and Richard W. Linck 1 1 University of Minnesota, Department of Cell Biology and Neuroanatomy, 321 Church St, Minneapolis, MN 55455, USA 2 Laboratory of Molecular Biology, Medical Research Center, Hills Road, Cambridge CB2 2QH, UK