A CELL-SPECIFIC GLYCOSYLATED SILK PROTEIN FROM CHIRONOMUS-THUMMI SALIVARY-GLANDS - CLONING, CHROMOSOMAL LOCALIZATION, AND CHARACTERIZATION OF CDNA

Chironomid salivary glands contain 40 cells dedicated to the synthesis of a relatively small ensemble of silk proteins. Glands in some speci es contain a special lobe composed of 4 cells distinguishable from the others, We have cloned a special lobe-specific cDNA from Chironomus t hummi salivary glands, Northern blots of salivary gland RNA demonstrat ed that the cDNA hybridizes to a 2.5-kilobase transcript present only in the special lobe. In situ hybridization mapped the gene encoding th is cDNA to region A2b on polytene chromosome TV, the locus of the spec ial lobe-specific Balbiani ring a, The deduced amino acid sequence enc odes a protein with a calculated molecular mass of 77 kDa and numerous potential glycosylation sites; it appears unrelated to other known ch ironomid silk proteins, Polyclonal antibody, raised against a cDNA-enc oded fusion protein, reacted exclusively with a special lobe-specific 160-kDa silk protein, Lectin binding studies indicate that the immunor eactive 160-kDa protein contains both N- and O-linked glycan moieties. We conclude that glycosylation most likely contributes to the differe nce between calculated and apparent molecular masses and that this cDN A encodes the special lobe-specific silk protein previously described as ssp160