Extraordinary conservation of cysteines among homologous Chironomus silk proteins sp185 and sp220

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.