Extraordinary Conservation of Cysteines Among Homologous Chironomus Silk Proteins sp185 and sp220

Abstract

Aquatic larvae of the midge, Chironomus tentans, synthesize a 185-kDa silk protein (sp185) with the cysteine-containing motif Cys-X-Cys-X-Cys (where X is any residue) every 20-28 residues. We report here the cloning and full-length sequence of cDNAs encoding homologous silk proteins from Chironomus pallidivittatus (sp185) and Chironomus thummi (sp220). Deduced amino acid sequences reveal proteins of nearly identical mass composed of 72 blocks of 20-28 residues, 61% of which can be described by the motif X5-8-Cys-X5-(Trp/Phe/Tyr)-X4-Cys-X-Cys-X-Cys. Spatial arrangement of these residues is preserved more than surrounding sequences. cDNA clones enabled us to map the genes on polytene chromosomes and identify for the first time the homolog of the Camptochironomus Balbiani ring 3 locus in Chironomus thummi. The apparent molecular weight difference between these proteins (185 vs 220 kDa) is not attributable to primary structure and may be due to differential N-linked glycosylation. DNA distances and codon substitutions indicate that the C. tentans and C. pallidivittatus genes are more related to each other than either is to C. thummi; however, substitution rates for the 5'- and 3'-halves of these genes are different. Blockwise sequence comparisons suggest intragenic variation in that some regions evolved slower or faster than the mean and may have been subjected to different selective pressures.

Population variability in Chironomus (Camptochironomus) species (Diptera, Nematocera) with a Holarctic distribution: evidence of mitochondrial gene flow

“…In previous studies (e.g. Kao et al ., 1994; Chen et al ., 1995; Case et al ., 1997; Gruhl et al ., 1997; Guryev et al ., 2001) a separation time of 60 My for the separation of the globin genes of C. thummi and C. tentans has been used as the basis for calculating separation times of other species. This figure was derived from the Goodman et al .…”

Cloning and Characterization of a Novel Mannose-binding Protein of Acanthamoeba

“…The homology among MBP, silk, and Balbiani ring proteins is because of the presence of these CXCXC repeats. This conserved pattern CXCXC, where X can be any amino acid, is found in up to five copies in vascular endothelial growth factor C (41) and over 70 copies in silk and Balbiani ring proteins (42). Whereas the function of the CX-CXC motifs remains to be determined, a similar motif, CXXC, is well known for its redox function in many proteins (43).…”

Adhesion in blue mussels (Mytilus edulis) and barnacles (genus Balanus): Mechanisms and technical applications

“…Since the 20 kD protein was recovered without reduction treatment, Cys residues were suggested to form not inter-but intramolecular disulfide bonds being responsible for the proposed compactness of the molecule. A Cys-Xaa-CysXaa-Cys motif occurring in one region of the 20 kD protein is also known from the 185 kD silk protein of the aquatic midge larvae Chironomus tentans, in which the motif is repeated after approximately 22 -26 residues (Case et al, 1997). The protein, used in the construction of larval tubes, has been proposed to prevent random aggregation of other components in the gland (cit.…”