A mosquito-specific protein family includes candidate receptors for malaria sporozoite invasion of salivary glands

Abstract

SummaryWe describe a previously unrecognized protein family from Aedes and Anopheles mosquitoes, here named SGS proteins. There are no SGS homologues in Drosophila or other eukaryotes, but SGS presence in two mosquito genera suggests that the protein family is widespread among mosquitoes. Ae. aegypti aaSGS1 mRNA and protein are salivary gland specific, and protein is localized in the basal lamina covering the anatomical regions that are preferentially invaded by malaria sporozoites. Anti-aaSGS1 antibodies inhibited sporozoite invasion into the salivary glands in vivo , confirming aaSGS1 as a candidate sporozoite receptor. By homology to aaSGS1 we identified the complete complement of four SGS genes in An. gambiae , which were not recognized in the genome annotation. Two An. gambiae SGS genes display salivary gland specific expression like aaSGS1 . Bioinformatic analysis predicts that SGS proteins possess heparin-binding domains, and have among the highest density of tyrosine sulphation sites of all An. gambiae proteins. The major sporozoite surface proteins (CS and TRAP) also bind heparin, and interact with sulphoconjugates during liver cell invasion. Thus, we speculate that sporozoite invasion of mosquito salivary glands and subsequently the vertebrate liver may share similar mechanisms based on sulphation. Phylogenomic analysis suggests that an SGS ancestor was involved in a lateral gene transfer.

Members of the Salivary Gland Surface Protein (SGS) Family Are Major Immunogenic Components of Mosquito Saliva

“…The only published study to date that focuses on the biology of SGSs showed that a member of the A. aegypti SGS gene family, Sgs1, is produced exclusively in the salivary glands, where it is embedded in the basal lamina and serves as an inadvertent receptor for P. gallinaceum sporozoites (18). In the present study, we found that A. gambiae Sgs4 and Sgs5 are also present on the basal side of the salivary glands, and because the major portions of these proteins are cleaved from their transmembrane domain, it is likely that they are also embedded in the basal lamina.…”

“…However, Western blots conducted using the 3-8% Tris-acetate polyacrylamide gels that are recommended for high molecular weight proteins, along with the HiMark TM high weight protein standard, showed that Sgs4 and Sgs5 are ϳ300 kDa (Fig. 6, A and B) instead of the ϳ220 kDa previously reported for A. aegypti Sgs1 (18). Western blots using 4 -12% BisTris and 4% Tris/glycine gels and two high mass standards confirmed the ϳ300 kDa mass of Sgs4 and Sgs5 (Fig.…”

“…A member of this gene family was independently identified while screening a panel of monoclonal antibodies produced against A. aegypti salivary gland surface proteins. This protein, named aaSgs1 (GenBank TM number AAV28546), was shown to be required for the successful invasion of A. aegypti salivary glands by Plasmodium gallinaceum sporozoites (18). Thus, the biology of SGSs is intriguing, because they have been implicated in the Plasmodium life cycle and represent a rare case of transdomain horizontal gene transfer.…”

“…During a transcriptomic analysis of female A. gambiae salivary glands, Arcà et al (16) identified a contiguous series of large genes of unknown function (Vector base IDs: ENSANGP00000027299, ENSANGP00000027791, and ENSANGP00000029569; later named SGS 2 by Korochkina et al (18)) that had not been predicted during earlier genome scans and that appear to have been horizontally transferred into the mosquito genome from a bacterial source whose only known living relatives are Wolbachia proteobacteria (19). A member of this gene family was independently identified while screening a panel of monoclonal antibodies produced against A. aegypti salivary gland surface proteins.…”

Extensive genomic diversity of closely related Wolbachia strains

“…WD_0513 is interesting as its closest putative homologues are found in the genomes of Anopheles gambiae, Aedes aegypti and Culex quinquefasciatus (Woolfit et al, 2009) and facilitate invasion of malarial sporozoites into salivary gland cells (Arca et al, 2005;Korochkina et al, 2006 (Dunning Hotopp et al, 2007;Kondo et al, 2002), but in the case of WD_0513, the actual direction of this transfer is difficult to ascertain without further knowledge of the gene's phylogenetic distribution (Woolfit et al, 2009). However, the limited distribution of this region and the presence of these proteins in both the endosymbiont and host make these genes interesting candidates for research into host-parasite interactions.…”