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.
Max Planck Institute for Biophysical Chemistry, Department of Biochemistry, Goettingen, Federal Republic of Germany.
K
K
Weber
N
N
Geisler
U
U
Plessmann
A
A
Bremerich
K F
KF
Lechtreck
M
M
Melkonian
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