cDNA for a cell-specific silk protein from Chironomus thummi

Chironomid salivary glands contain thirty-five cells dedicated to the synthesis of a relatively small ensemble of silk proteins. Glands in some species contain a special lobe composed of four cells distinguishable from the others. I have cloned a special lobe-specific cDNA from Chironomus thummi salivary glands. Northern blots of salivary gland RNA demonstrated that the cDNA hybridizes to a 2.5-kb transcript present only in the special lobe. In situ hybridization mapped the gene encoding this cDNA to region A2b on polytene chromosome IV, the locus of the special lobe-specific Balbiani ring a. The deduced amino acid sequence encodes a protein with a calculated molecular weight of 77 kDa and numerous potential glycosylation sites which appears unrelated to other known chironomid silk proteins. A polyclonal antibody, raised against a cDNA-encoded fusion protein, reacted exclusively with a special lobe-specific 160-kDa silk protein. Lectin binding to Western blots suggests that this protein contains both N- and O-linked carbohydrate moieties. I conclude that glycosylation contributes to the difference between calculated and apparent molecular weights and that this cDNA encodes the special lobe-specific silk protein previously defined as ssp160.