The Structural Genes for Three Drosophila Glue Proteins Reside at a Single Polytene Chromosome Puff Locus

Abstract

The polytene chromosome puff at 68C on the Drosophila melanogaster third chromosome is thought from genetic experiments to contain the structural gene for one of the secreted salivary gland glue polypeptides, sgs-3. Previous work has demonstrated that the DNA included in this puff contains sequences that are transcribed to give three different polyadenylated RNAs that are abundant in third-larval-instar salivary glands. These have been called the group II, group III, and group IV RNAs. In the experiments reported here, we used the nucleotide sequence of the DNA coding for these RNAs to predict some of the physical and chemical properties expected of their protein products, including molecular weight, amino acid composition, and amino acid sequence. Salivary gland polypeptides with molecular weights similar to those expected for the 68C RNA translation products, and with the expected degree of incorporation of different radioactive amino acids, were purified. These proteins were shown by amino acid sequencing to correspond to the protein products of the 68C RNAs. It was further shown that each of these proteins is a part of the secreted salivary gland glue: the group IV RNA codes for the previously described sgs-3, whereas the group II and III RNAs code for the newly identified glue polypeptides sgs-8 and sgs-7.

Drosophila ELL is associated with actively elongating RNA polymerase II on transcriptionally active sites in vivo

“…These results demonstrate that endogenous dELL (Figure 4A) is widely dispersed on developmental puff sites, which are generally recognized as sites of active transcription (Pelling, 1964; Plagens et al ., 1976). Among the prominent dELL binding sites are the loci encoding the salivary gland glue proteins sgs 3, 4, 5, 7 and 8, which are highly transcribed in mid‐third instar larval salivary glands (Figure 4A; Korge, 1975; Guild and Shore, 1984; Crowley et al ., 1983).…”

A novel cysteine-rich adaptor protein is required for mucin packaging and secretory granule stability in vivo

“…Sgs7 encodes a small unglycosylated protein (~8 kDa predicted molecular mass) that is co-expressed with the highly glycosylated Sgs3 (~32 kDa predicted molecular mass as an unglycosylated monomer) during the third instar larval period in response to ecdysone pulses ( 31 – 34 ). To understand how Sgs7 may be affecting the folded structure of Sgs3, we examined where each protein resides within WT SGs.…”

“…Interestingly, we found loss of Sgs7 disrupted the organized structures of one specific mucin, Sgs3, within secretory granules, while the restructuring of other mucins was unaffected. Sgs3 and Sgs7 form a tightly linked cluster on the 3L chromosomal arm, suggesting that there has been evolutionary pressure to ensure the co-expression of these two proteins ( 34 , 36 , 37 ). This further suggests that there may be other adaptors that are specific for different mucins.…”