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 species contain a special lobe composed of 4 cells distinguishable from the others. We 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-kilobase 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 mass of 77 kDa and numerous potential glycosylation sites; it appears unrelated to other known chironomid silk proteins. Polyclonal antibody, raised against a cDNA-encoded fusion protein, reacted exclusively with a special lobe-specific 160-kDa silk protein. Lectin binding studies indicate that the immunoreactive 160-kDa protein contains both N- and O-linked glycan moieties. We conclude that glycosylation most likely contributes to the difference between calculated and apparent molecular masses and that this cDNA encodes the special lobe-specific silk protein previously described as ssp160 (Kolesnikov, N. N., Karakin, E. I., Sebeleva, T. E., Meyer, L., and Serfling, E. (1981) Chromosoma 83, 661-677).