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

Abstract

Blood-type B specific hemagglutinin (L-rham/D-gal-specific lectin) secreted by Streptomyces sp. 27S5 (SHA) was purified over 40 years ago. Despite the loss of the original strain and the lack of genomic information, the amino acid sequence of its SHA was recently determined based on peptides matching to a hypothetical protein of S. lavendulae, differing only by a single amino acid, A108E (J Biol Chem 293:368, 2018). Recombinant SHA (rSHA A108E) exhibited binding to L-rham on Lactobacillus casei (Shirota) and blood type B-specificity, supporting the notion that rSHA A108E is equivalent to the authentic SHA. To address questions how SHA was biosynthesized and secreted by the lost strain as well as what biological roles SHA may have, we needed to identify SHA-secreting S. strains that are identical to or at least comparable to the lost strain. We took advantage of the IMC's collection of over 40,000 actinomycete strains. Out of 5,000 strains with available 16S rRNA gene signatures, 67 strains shared significant 16S rRNA sequence homology with S. lavendulae, and were examined. We found that 17 S. lavendulae-related strains secreted SHA homologues in cultures. Seven SHA homologues were purified to homogeneity by gum arabic affinity chromatography. All homologues were secreted as pro-SHA proteins having larger masses than archived SHA. Proteolytic processing of the pro-SHAs occurred during and after purification, which indicated that associated proteases apparently converted pro-SHAs into mature SHAs with molecular masses similar to that of the archived SHA. Pro-SHAs did not exhibit hemagglutination (HA) activity, but bound to gum arabic gels, suggesting that pro-SHAs have one active glycan binding site. The processed SHA proteins appear to possess specific HA activities equivalent to that of the archived SHA. Comparison of strain-specific 16S rRNA and SHA sequences revealed phylogenetic inconsistencies, which cannot be explained by the deletion and horizontal transmission repeated in this phylogenic group alone. Genome analysis of 1,234 Streptomyces strains from the database resulted in the discovery of 18 strains encoding SHA genes. Of those, 15 strains were newly identified and 16 strains revealed a unique syntenic region which encodes the SHA gene, supporting limited propagation of SHA in the course of evolution. In short, we succeeded in purifying SHA homologues from seven strains. The lost strain Streptomyces sp. 27S5, however, was exceptional, because it expressed and secreted a processed form of SHA protein at levels that were at least 4 times higher than those of the two best-SHA homologue-producing strains. Apart from structural and functional studies on homologous SHA proteins, this study led to an intriguing discovery, i.e., the identification of SHA gene in syntenic regions. It is plausible that SHA would play a role in the unknown function achieved by the ensemble of accompanying genes in synteny.