Abstract
ABSTRACTSerine-rich repeat proteins (SRRPs) have emerged as an important group of cell surface adhesins found in a growing number of Gram-positive bacteria. Studies focused on SRRPs from streptococci and staphylococci demonstrated that these proteins are O-glycosylated on serine or threonine residues and exported via an accessory secretion (aSec) system. In pathogens, these adhesins contribute to disease pathogenesis and represent therapeutic targets. Recently, the non-canonical aSec system has been identified in the genomes of gut microbes and characterization of their associated SRRPs is beginning to unfold, showing their role in mediating attachment and biofilm formation. Here we provide an update of the occurrence, structure, and function of SRRPs across bacteria, with emphasis on the molecular and biochemical properties of SRRPs from gut symbionts, particularly Lactobacilli. These emerging studies underscore the range of ligands recognized by these adhesins and the importance of SRRP glycosylation in the interaction of gut microbes with the host.
Highlights
SRRPs cover a functionally and structurally diverse family of glycoproteins found in many Grampositive bacteria[1]
Export of SRRPs onto the bacterial surface occurs through a dedicated non-canonical Sec translocase, SecY2A2.21 This accessory secretion system is encoded by genes that are normally co-located with the srrp gene(s) within a gene cluster and is composed of the motor protein SecA2, the translocon channel SecY2 and three to five accessory Sec proteins (Asp15)
This study showed that the L. reuteri ATCC 53608 and 100-23C strains were capable of performing protein glycosylation and that SRRP100-23 and SRRP53608 were glycosylated with Hex-HexHexNAc and di-HexNAc moieties, respectively
Summary
SRRPs cover a functionally and structurally diverse family of glycoproteins found in many Grampositive bacteria[1]. The SRRPs found in many strains of the pathogenic species Streptococcus suis have an exceptionally large BR domain that can contain motifs such as a bacterial group 3 immunoglobulin (Ig)-like domain (Figure S2B) or a FlgD Tudor-like domain (Table S2) These putative binding domains in addition to the well-characterized BR domains may contribute to the binding of SRRPs to a variety of ligands. Crystal structures of seven SRRP-BRs have been reported for Grampositive pathogens to date, highlighting a relationship between their structural folds and binding ligands (Table 2) The presence of complete SecA2/Y2 clusters with an intact SRRP in the genomes of other Lactobacillus species (see above), suggest a common role of SRR glycoproteins in adhesion to host epithelia, which may be related to the ecological context of these strains (see Duar et al, 201788 for a review)
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