The cruciated microtubule-associated fibers of the green alga Dunaliella bioculata consist of a 31 kDa SF-assemblin.

Cytoskeletons of Dunaliella bioculata, the biflagellate wallless green alga, were isolated and analyzed using a monoclonal and a polyclonal antibody raised against SF-assemblin, the major protein of the two striated microtubule-associated fibers of the alga Spermatozopsis similis. Indirect immunofluorescence showed antigenic structures associated with the four microtubular flagellar roots. SDS-PAGE followed by immunoblot analysis revealed a cross-reacting polypeptide of 31 kDa. This protein of D. bioculata was isolated using gel filtration chromatography in 8 M urea and in vitro reassembly of striated fibers. Microsequencing of the purified protein yielded various peptides, which could be aligned along the sequence of SF-assemblin from S. similis. A complete sequence of the Dunaliella protein was obtained by cDNA cloning. It documents the non helical head domain followed by a helical rod domain with a 29 residue repeat pattern based on four heptads followed by a skip residue. Compared to SF-assemblin of S. similis the SF-assemblin of Dunaliella has a shorter head and a slightly longer rod domain. The two algal SF-assemblins share only 57% sequence identity. We conclude that SF-assemblin and related proteins in various protists are representatives of a new class of alpha-helical proteins characterized by the ability to form a special segmented coiled coil and to assemble into striated fibers of 2 nm protofilaments in vivo and in vitro.