Proteolytic Processing, Maturation, and Unique Synteny of the Streptomyces Hemagglutinin SHA

Abstract

ABSTRACT SHA is an l-rhamnose- and d-galactose-binding lectin that agglutinates human group B erythrocytes and was first purified almost 50 years ago. Although the original SHA-producing Streptomyces strain was lost, the primary structure of SHA was more recently solved by mass spectrometry of the archived protein, which matched it to a similar sequence in the Streptomyces lavendulae genome. Using genomic and protein biochemical analyses, this study aimed to identify SHA-secreting Streptomyces strains to further investigate the expression and binding activities of these putative proteins. Of 67 strains genetically related to S. lavendulae, 17 secreted pro-SHAs in culture. Seven SHA homologues were purified to homogeneity and then subjected to liquid chromatography–high-resolution multistage mass spectrometry (LC-MS/MS) and hemagglutination (HA) assays. Processing of pro-SHAs occurred during and after purification, indicating that associated proteases converted pro-SHAs into mature SHAs with molecular masses and HA activities similar to that of the archived SHA. Previously, the SHA monomer was shown to have two carbohydrate binding sites. The present study, however, found no HA activity in pro-SHAs, suggesting that pro-SHAs have only one binding site. Genetically, the SHA gene resides in conserved syntenic regions. The published genomes of 1,234 Streptomyces strains were analyzed, revealing 18 strains with SHA genes, 16 of which localized to a unique syntenic region. The SHA syntenic region consists of ∼17 open reading frames (ORFs) and is specific to S. lavendulae-related strains. Notably, a lipoprotein gene excludes SHA from the synteny in some strains, suggesting that horizontal gene transfer events during the course of evolution shaped the distribution of SHA genes. IMPORTANCE Lectins are extremely useful molecules for the study of glycans and carbohydrates. Here, we show that homologous genes encoding the l-rhamnose- and d-galactose-binding lectins, SHAs, are present in multiple bacterial strains, genetically related to Streptomyces lavendulae. SHA genes are expressed as precursor pro-SHA proteins that are truncated and mature into fully active lectins with two carbohydrate binding sites, which exhibit hemagglutination activity for type B red blood cells. The SHA gene is located within a conserved syntenic region, hinting at specific but yet-to-be-discovered biological roles of this carbohydrate-binding protein for its soil-dwelling microbial producer.

Keywords

• microbial lectin
• Streptomyces
• blood type B-specific hemagglutination
• lectin synteny