Larval salivary gland secretion proteins in Drosophila structural analysis of the Sgs-5 gene

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

Mucins are large, highly glycosylated extracellular matrix proteins that line and protect epithelia of the respiratory, digestive, and urogenital tracts. Previous work has shown that mucins form large, interconnected polymeric networks that mediate their biological functions once secreted. However, how these large matrix molecules are compacted and packaged into much smaller secretory granules within cells prior to secretion is largely unknown. Here, we demonstrate that a small cysteine-rich adaptor protein is essential for proper packaging of a secretory mucin in vivo. This adaptor acts via cysteine bonding between itself and the cysteine-rich domain of the mucin. Loss of this adaptor protein disrupts mucin packaging in secretory granules, alters the mobile fraction within granules, and results in granules that are larger, more circular, and more fragile. Understanding the factors and mechanisms by which mucins and other highly glycosylated matrix proteins are properly packaged and secreted may provide insight into diseases characterized by aberrant mucin secretion.

Drosophila Glue: A Promising Model for Bioadhesion

The glue produced by Drosophila larvae to attach themselves to a substrate for several days and resist predation until the end of metamorphosis represents an attractive model to develop new adhesives for dry environments. The adhesive properties of this interesting material have been investigated recently, and it was found that it binds as well as strongly adhesive commercial tapes to various types of substrates. This glue hardens rapidly after excretion and is made of several proteins. In D. melanogaster, eight glue proteins have been identified: four are long glycosylated mucoproteins containing repeats rich in prolines, serines and threonines, and four others are shorter proteins rich in cysteines. This protein mix is produced by the salivary glands through a complex packaging process that is starting to be elucidated. Drosophila species have adapted to stick to various substrates in diverse environmental conditions and glue genes appear to evolve rapidly in terms of gene number, number of repeats and sequence of the repeat motifs. Interestingly, besides its adhesive properties, the glue may also have antimicrobial activities. We discuss future perspectives and avenues of research for the development of new bioadhesives mimicking Drosophila fly glue.

Evolution of salivary glue genes in Drosophila species

Background

At the very end of the larval stage Drosophila expectorate a glue secreted by their salivary glands to attach themselves to a substrate while pupariating. The glue is a mixture of apparently unrelated proteins, some of which are highly glycosylated and possess internal repeats. Because species adhere to distinct substrates (i.e. leaves, wood, rotten fruits), glue genes are expected to evolve rapidly.

Results

We used available genome sequences and PCR-sequencing of regions of interest to investigate the glue genes in 20 Drosophila species. We discovered a new gene in addition to the seven glue genes annotated in D. melanogaster. We also identified a phase 1 intron at a conserved position present in five of the eight glue genes of D. melanogaster, suggesting a common origin for those glue genes. A slightly significant rate of gene turnover was inferred. Both the number of repeats and the repeat sequence were found to diverge rapidly, even between closely related species. We also detected high repeat number variation at the intrapopulation level in D. melanogaster.

Conclusion

Most conspicuous signs of accelerated evolution are found in the repeat regions of several glue genes.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1364-9) contains supplementary material, which is available to authorized users.

A Drosophila salivary gland mucin is also expressed in immune tissues: evidence for a function in coagulation and the entrapment of bacteria

Our studies on the developmental regulation of glycosylation in Drosophila melanogaster led us to identify and characterize gp150, an ecdysone-regulated mucin that is found in hemocytes, the gut (peritrophic membrane) and in the salivary glands. We are particularly interested in mucin immune functions and found that gp150 is released from larval hemocytes, becomes part of the clot and participates in the entrapment of bacteria. By RT-PCR and RNAi experiments, we identified gp150 as the previously described I71-7, an ecdysone-induced salivary glue protein. We discuss the evolutionary and biochemical implications of the dual use of salivary proteins for immune functions in insects. Further molecular characterization of such shared proteins may enable a better understanding of the properties of proteins involved in containment and elimination of microbes, as well as hemostasis and wound repair.

Molecular cloning of the Drosophila virilis larval glue protein gene Lgp-3 and its comparative analysis with other Drosophila glue protein genes

DNA comprising the larval glue protein gene Lgp-3 of Drosophila virilis was isolated from a lambda genomic and a cDNA library. The transcription start site, two polyadenylation sites and the boundaries of the single intron were determined. An open reading frame encoding 379 amino acids was found. At the DNA level the presence of similar introns and three conserved sequence motifs in the proximal promoters suggest that the gene is related to those of the D. virilis lgp-1 and the D. melanogaster sgs-3, -7 and -8 glue proteins. Their common ancestry is also substantiated by the comparisons of the deduced amino acid sequences and the profiles of hydropathic indices, which reveal striking similarities of the N- and C-termini and of the central repeat domains, although the lengths and the primary structures of the proteins diverged considerably during 60 million years of separate evolution of the two Drosophila species.

A transcriptional switch between the Pig-1 and Sgs-4 genes of Drosophila melanogaster

Pig-1 and Sgs-4 are a pair of closely linked and divergently transcribed Drosophila melanogaster genes, which are both expressed in larval salivary glands but at different times during development. While Sgs-4 is expressed at high levels only at the end of the third instar, Pig-1 exhibits a major peak of expression during late second and early third instar. Thus, Pig-1 expression declines as Sgs-4 expression is induced. In this paper, we show that three adjacent elements located within the short region between these genes can account for the switch from Pig-1 to Sgs-4 expression. A 170-bp segment acts as an enhancer to direct Sgs-4 expression in late-third-instar salivary glands. A 64-bp sequence located just upstream from the enhancer can modify its temporal specificity so that it works throughout the third instar. Expression induced at mid-third instar by a combination of these two elements can be repressed by a negative regulatory sequence located still further upstream. We present evidence suggesting that the changing interactions between these regulatory elements and the Sgs-4 and Pig-1 promoters lead to the correct pattern of expression of the two genes.

A transcriptional switch between the Pig-1 and Sgs-4 genes of Drosophila melanogaster

Pig-1 and Sgs-4 are a pair of closely linked and divergently transcribed Drosophila melanogaster genes, which are both expressed in larval salivary glands but at different times during development. While Sgs-4 is expressed at high levels only at the end of the third instar, Pig-1 exhibits a major peak of expression during late second and early third instar. Thus, Pig-1 expression declines as Sgs-4 expression is induced. In this paper, we show that three adjacent elements located within the short region between these genes can account for the switch from Pig-1 to Sgs-4 expression. A 170-bp segment acts as an enhancer to direct Sgs-4 expression in late-third-instar salivary glands. A 64-bp sequence located just upstream from the enhancer can modify its temporal specificity so that it works throughout the third instar. Expression induced at mid-third instar by a combination of these two elements can be repressed by a negative regulatory sequence located still further upstream. We present evidence suggesting that the changing interactions between these regulatory elements and the Sgs-4 and Pig-1 promoters lead to the correct pattern of expression of the two genes.

Comparison of several promoters and polyadenylation signals for use in heterologous gene expression in cultured Drosophila cells

We have directly compared the ability of four promoters and three polyadenylation (poly(A)) signals to direct heterologous gene expression in stably transfected Drosophila melanogaster S2 cells. We compared two constitutive Drosophila promoters, the actin 5C distal promoter and the alpha 1-tubulin promoter, with the tightly regulated Drosophila metallothionein (Mtn) promoter and the Bombyx mori fibroin promoter. We find that the actin 5C and induced Mtn promoters generate comparable high levels of RNA and protein in this system. The alpha 1-tubulin promoter generates about four-fold lower levels, and the fibroin promoter shows no detectable activity in S2 cells. Interestingly, genes expressed from the constitutive actin 5C and alpha 1-tubulin promoters are consistently present at three- to four-fold lower copy numbers than genes expressed from the inducible Mtn promoter or the inactive fibroin promoter. Poly(A) signals of both mammalian (SV40) and Drosophila (Mtn) origin efficiently directed stable RNA synthesis in S2 cells, and, as in mammalian cells, the SV40 late poly(A) signal was more efficient than the SV40 early poly(A) signal. Thus the process of polyadenylation appears to be conserved between mammalian and Drosophila cells.

Perturbation-reperturbation test of selection vs. hitchhiking of the two major alleles of Esterase-5 in Drosophila pseudoobscura

A perturbation-reperturbation tests selective neutrality of 100/100/100/100/100 and 106/100/100/100/100, the two most common alleles at the highly polymorphic X-linked locus Esterase-5 in Drosophila pseudoobscura. A total of 22 replicate populations are set up in cages, 11 start at a high frequency of 76% (U) and 11 at a low frequency of 21% (N) of the 106 allele. Allele frequencies change directionally and decrease in both U and N populations as groups and reach equilibria of 60 and 14%, respectively, after 200-300 days. These changes suggest natural selection. A hypothesis of balancing selection accounts for the pattern and predicts a dynamic equilibrium. A rival neutral hypothesis accounts for the pattern equally well by postulating hitchhiking and breakup of linkage leaving the Est-5 variants to drift at neutral equilibria. A reperturbation of allele frequencies in each population, creating 22 additional reperturbed populations EN and EU, with the original populations as controls, directly addresses the question of balancing selection or hitchhiking and breakup of linkage effects. Allele frequencies do not change directionally among the reperturbed populations as a group. The hypothesis of balancing selection is rejected in favor of the hypothesis of initial hitchhiking and dissipated linkage effects. The power of the experimental design to detect selection is studied by simulation. Within the limits of power set by the design, it is concluded that the 100 and 106 are iso-fitness alleles of Est-5 under the environmental conditions of the laboratory populations. The requirements of a method of perturbation and reperturbation are discussed.

cis-acting sequences required for expression of the divergently transcribed Drosophila melanogaster Sgs-7 and Sgs-8 glue protein genes

The Sgs-7 and Sgs-8 glue genes at 68C are divergently transcribed and are separated by 475 bp. Fusion genes with Adh or lacZ coding sequences were constructed, and the expression of these genes, with different amounts of upstream sequences present, was tested by a transient expression procedure and by germ line transformation. A cis-acting element for both genes is located asymmetrically in the intergenic region between -211 and -43 bp relative to Sgs-7. It is required for correct expression of both genes. This element can confer the stage- and tissue-specific expression pattern of glue genes on a heterologous promoter. An 86-bp portion of the element, from -133 to -48 bp relative to Sgs-7, is shown to be capable of enhancing the expression of a truncated and therefore weakly expressed Sgs-3 fusion gene. Recently described common sequence motifs of glue gene regulatory elements (T. Todo, M. Roark, K. Vijay Raghavan, C. A. Mayeda, and E.M. Meyerowitz, Mol. Cell. Biol. 10:5991-6002, 1990) are located within this 86-bp region.

cis-acting sequences required for expression of the divergently transcribed Drosophila melanogaster Sgs-7 and Sgs-8 glue protein genes

The Sgs-7 and Sgs-8 glue genes at 68C are divergently transcribed and are separated by 475 bp. Fusion genes with Adh or lacZ coding sequences were constructed, and the expression of these genes, with different amounts of upstream sequences present, was tested by a transient expression procedure and by germ line transformation. A cis-acting element for both genes is located asymmetrically in the intergenic region between -211 and -43 bp relative to Sgs-7. It is required for correct expression of both genes. This element can confer the stage- and tissue-specific expression pattern of glue genes on a heterologous promoter. An 86-bp portion of the element, from -133 to -48 bp relative to Sgs-7, is shown to be capable of enhancing the expression of a truncated and therefore weakly expressed Sgs-3 fusion gene. Recently described common sequence motifs of glue gene regulatory elements (T. Todo, M. Roark, K. Vijay Raghavan, C. A. Mayeda, and E.M. Meyerowitz, Mol. Cell. Biol. 10:5991-6002, 1990) are located within this 86-bp region.

Molecular characterization of DNA puff II/9A genes in Sciara coprophila

A cDNA clone, pSDII/9, that hybridizes in situ to ecdysone-regulated DNA puff II/9A in Sciara coprophila was used as a probe to isolate a Sciara genomic clone. lambda pSDII/9, which contains a 14.7 x 10(3) base-pair DNA insert. The full-length cDNA insert was sequenced and mapped to gene II/9-1 on the genomic clone. A second gene (II/9-2), transcribed in the same direction as II/9-1, was also mapped to lambda pSDII/9, and its nucleic acid sequence was found to be 85% similar to that of gene II/9-1. An RNase protection assay demonstrates that gene II/9-1 contains a single intron that also exists in gene II/9-2 according to sequencing analysis and primer extensions of RNA encoded by this gene. Computer analyses of the deduced amino acid sequences of genes II/9-1 and II/9-2 indicate that the two DNA puff-encoded proteins are mostly alpha-helical coiled-coils. The 5'-flanking sequences of both genes contain regions that are similar to other ecdysone-regulated genes from Drosophila melanogaster.

Regulatory elements and interactions in the Drosophila 68C glue gene cluster

We reviewed studies on the developmental regulation of the 68C glue gene cluster of Drosophila melanogaster. Extensive transformation analyses of Sgs-3 have shown that four regions necessary for normal expression can be distinguished. The first(+10 to -50) contains the transcription start site and TATA motif. This region can be replaced functionally by corresponding sequences from the hsp70 gene, but it is sensitive to point mutations in the TATA sequence. The second region (-50 to -98) contains more than one upstream sequence that, in combination with the other elements, leads to stage and tissue-specific expression. The third region (centered at -600) contains an element that enhances transcript levels some 20-fold. The final region (between -1.65 and -2.35 kb) contains elements having modest (twofold to threefold) effects on expression, one of which is contained in the coding sequences of Sgs-7, a second member of the cluster.

Two proteins from Drosophila nuclei are bound to chromatin and are detected in a series of puffs on polytene chromosomes

Immunizing chromatin protein fractions from Drosophila melanogaster embryos, monoclonal antibodies have been generated against two nuclear proteins of different molecular weight. These proteins are present in a chromatin fraction of Drosophila Kc-cell nuclei and both proteins could be shown to cosediment with nucleosomes following separation on sucrose gradients. Early in development both proteins are located in the embryo cytoplasm. Later, at times when transcription starts at blastoderm, they become redistributed into the nuclei. On salivary gland chromosomes both proteins are detected in a series of developmentally active puffs. The number of sites where these antigens can be detected, as well as the qualitative properties of the antigens at these sites differ between both proteins.

Closely linked DNA elements control the expression of the Sgs-5 glue protein gene in Drosophila

cis-acting sequence regions involved in the regulation of Sgs-5 gene expression were mapped by testing DNA segments containing the Sgs-5 RNA coding region and various amounts of adjacent sequences for the ability to express Sgs-5 RNA. Following injection of the DNA segments into Drosophila embryos, expression of the gene was assayed in the salivary glands of the injected animals after they developed to third instar larvae, these somatically transformed individuals serving as an in vivo transient expression system. The information necessary for the expression of Sgs-5 is contained within 109 bp upstream and 69 bp downstream of the transcribed region. Somatic transformation experiments also show that some feature within the limits of a 1012-bp DNA segment containing the Sgs-5 RNA coding region derived from the Sgs-5 RNA null stock CA-2 must be responsible for the lack of transcription from this allele. The only DNA sequence differences between active and null alleles, within the 1012 bp, are seven single-base-pair substitutions between -84 bp and +175 bp relative to the RNA start site. One or a combination of these sites are likely contributors to the transcriptional inactivity of the Sgs-5CA2 allele.