SF-assemblin, the structural protein of the 2-nm filaments from striated microtubule associated fibers of algal flagellar roots, forms a segmented coiled coil

• Published in: The Journal of cell biology Vol. 121; no. 4; pp. 837 - 845
• Main Authors: Weber, K. (Max Planck Institute for Biological Chemistry, Goettingen, FRG), Geisler, N, Plessmann, U, Bremerich, A, Lechtreck, K.F, Melkonian, M
• Format: Journal Article
• Language: English
• Published: New York, NY Rockefeller University Press 01.05.1993; The Rockefeller University Press
• Subjects: Algae; Amino Acid Sequence; amino acid sequences; Amino acids; Aquatic plants; Biochemistry; Biological and medical sciences; Cell structures and functions; Cellular biology; chemistry; CHLOROPHYCEAE; Chlorophyta; Chlorophyta - chemistry; Chlorophyta - ultrastructure; Circular Dichroism; Cytoskeleton, cytoplasm. Intracellular movements; Dimers; Disulfides; Electrophoresis, Polyacrylamide Gel; Endopeptidases; Endopeptidases - chemistry; ESTRUCTURA CELULAR; Fibers; flagellum; Fundamental and applied biological sciences. Psychology; Gels; Giardia lamblia; Microtubules; Microtubules - chemistry; Molecular and cellular biology; molecular conformation; Molecular Sequence Data; Molecular weight; Molecules; Periodicity; plant proteins; Protein Structure, Secondary; PROTEINAS VEGETALES; PROTEINE VEGETALE; Proteins; Sequence Homology, Amino Acid; Sequencing; Serine Endopeptidases; Spermatozopsis similis; STRUCTURE CELLULAIRE; ultrastructure; Proteins; Filament; Molecular structure; Algae; Cytoskeleton; Microtubule; Aminoacid sequence; Thallophyta
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.121.4.837

The microtubule associated system I fibers of the basal apparatus of the 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 component 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 rod domain of 253 residues is alpha-helical and seems to form a segmented coiled coil with a 29-residue repeat pattern based on four heptads followed by a skip residue. The distinct cluster of acidic residues at the COOH-terminal end of the motifs (periodicity about 4 nm) may be related to tubulin binding of SF-assemblin and/or its self assembly. A similar structure has been predicted from cDNA cloning of beta-giardin, a protein of the complex microtubular apparatus of the sucking disc in the protozoan flagellate Giardia lamblia. Although the rod domains of SF-assemblin and beta-giardin share only 20% sequence identity, they have exactly the same length and display 42% sequence similarity. These results predict that system I fibers and related microtubule associated structures arise from molecules able to form a special segmented coiled coil which can pack into 2-nm filaments. Such molecules seem subject to a strong evolutionary drift in sequence but not in sequence principles and length. This conservation of molecular architecture may have important implications for microtubule binding