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 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.
Weber
K
K
Max Planck Institute for Biophysical Chemistry, Department of Biochemistry, Goettingen, Federal Republic of Germany.
Geisler
N
N
Plessmann
U
U
Bremerich
A
A
Lechtreck
K F
KF
Melkonian
M
M
D000595:Amino Acid Sequence; D000460:Chlorophyta / Q000737:chemistry* / Q000648:ultrastructure; D002942:Circular Dichroism; D004591:Electrophoresis, Polyacrylamide Gel; D010450:Endopeptidases / Q000737:chemistry*; D008870:Microtubules / Q000737:chemistry*; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases
D010450:Endopeptidases; D012697:Serine Endopeptidases; C071910:assemblin