SF-ASSEMBLIN, THE STRUCTURAL PROTEIN OF THE 2-NM FILAMENTS FROM STRIATED MICROTUBULE-ASSOCIATED FIBERS OF ALGAL FLAGELLAR ROOTS, FORMS A SEGMENTED COILED-COIL

The microtubule associated system I fibers of the basal apparatus of t he flagellate green alga Spermatozopsis similis are noncontractile and display a 28-nm periodicity. Paracrystals with similar periodicities are formed in vitro by SF-assemblin, which is the major protein compon ent of system I fibers. We have determined the amino acid sequence of SF-assemblin and show that it contains two structural domains. The NH2 -terminal 31 residues form a nonhelical domain rich in proline. The ro d domain of 253 residues is alpha-helical and seems to form a segmente d coiled coil with a 29-residue repeat pattern based on four heptads f ollowed by a skip residue. The distinct cluster of acidic residues at the COOH-terminal end of the motifs (periodicity about 4 nm) may be re lated to tubulin binding of SF-assemblin and/or its self assembly. A s imilar structure has been predicted from cDNA cloning of beta-giardin, a protein of the complex microtubular apparatus of the sucking disc i n the protozoan flagellate Giardia lamblia. Although the rod domains o f SF-assemblin and beta-giardin share only 20% sequence identity, they have exactly the same length and display 42% sequence similarity. The se results predict that system I fibers and related microtubule associ ated structures arise from molecules able to form a special segmented coiled coil which can pack into 2-nm filaments. Such molecules seem su bject to a strong evolutionary drift in sequence but not in sequence p rinciples and length. This conservation of molecular architecture may have important implications for microtubule binding.