Molecular Determinants of Substrate Selectivity of a Pneumococcal Rgg-Regulated Peptidase-Containing ABC Transporter

Abstract

Peptidase-containing ABC transporters (PCATs) are a widely distributed family of transporters which secrete double-glycine (GG) peptides. In the opportunistic pathogen Streptococcus pneumoniae (pneumococcus), the PCATs ComAB and BlpAB have been shown to secrete quorum-sensing pheromones and bacteriocins related to the competence and pneumocin pathways. Here, we describe another pneumococcal PCAT, RtgAB, encoded by the rtg locus and found intact in 17% of strains. The Rgg/SHP-like quorum-sensing system RtgR/S, which uses a peptide pheromone with a distinctive Trp-X-Trp motif, regulates expression of the rtg locus and provides a competitive fitness advantage in a mouse model of nasopharyngeal colonization. RtgAB secretes a set of coregulated rtg GG peptides. ComAB and BlpAB, which share a substrate pool, do not secrete the rtg GG peptides. Similarly, RtgAB does not efficiently secrete ComAB/BlpAB substrates. We examined the molecular determinants of substrate selectivity between ComAB, BlpAB, and RtgAB and found that the GG peptide signal sequences contain all the information necessary to direct secretion through specific transporters. Secretion through ComAB and BlpAB depends largely on the identity of four conserved hydrophobic signal sequence residues previously implicated in substrate recognition by PCATs. In contrast, a motif situated at the N-terminal end of the signal sequence, found only in rtg GG peptides, directs secretion through RtgAB. These findings illustrate the complexity in predicting substrate-PCAT pairings by demonstrating specificity that is not dictated solely by signal sequence residues previously implicated in substrate recognition. IMPORTANCE The export of peptides from the cell is a fundamental process carried out by all bacteria. One method of bacterial peptide export relies on a family of transporters called peptidase-containing ABC transporters (PCATs). PCATs export socalled GG peptides which carry out diverse functions, including cell-to-cell communication and interbacterial competition. In this work, we describe a PCAT-encoding genetic locus, rtg, in the pathogen Streptococcus pneumoniae (pneumococcus). The rtg locus is linked to increased competitive fitness advantage in a mouse model of nasopharyngeal colonization. We also describe how the rtg PCAT preferentially secretes a set of coregulated GG peptides but not GG peptides secreted by other pneumococcal PCATs. These findings illuminate a relatively understudied part of PCAT biology: how these transporters discriminate between different subsets of GG peptides. Ultimately, expanding our knowledge of PCATs will advance our understanding of the many microbial processes dependent on these transporters. . Molecular determinants of substrate selectivity of a pneumococcal Rgg-regulated peptidasecontaining ABC transporter. mBio 11:e02502-19.

Quorum Sensing in Streptococcus mutans Regulates Production of Tryglysin, a Novel RaS-RiPP Antimicrobial Compound

“…1), and addition of this short peptide to reporter strains resulted in PdrA-dependent upregulation of promoter targets and autoinduction of SHP ( Fig. 2 and 4) (9,11,12,14,15). Also, similar to previously described SHP/Rgg systems, the PptAB and Opp transporters are required for the import and export of the SHP peptide, respectively (see Fig.…”

“…Also, similar to previously described SHP/Rgg systems, the PptAB and Opp transporters are required for the import and export of the SHP peptide, respectively (see Fig. S2 in the supplemental material) (9,11,12,50). Induction of this operon presumably results in the production of the S. mutans tryglysin, although detection of this peptide in CDM supernatants has eluded us so far.…”

“…Rgg regulators sense and respond to SHPs (short hydrophobic peptides) by directly binding these linear peptides, which induce allosteric changes in Rgg transcriptional activity ( 7 , 9 – 16 ). Rgg proteins have been characterized in various streptococcal species, including the medically relevant Streptococcus pyogenes ( 17 , 18 ), Streptococcus pneumoniae ( 11 – 13 ), and Streptococcus mutans ( 19 – 21 ). This family of proteins were originally identified in Streptococcus gordonii , as regulators of glucosyltransferase activity, and as such were named Rgg for r egulator g ene of g lucosyltransferase ( 22 , 23 ).…”

The pneumococcal social network

“…The best characterized pneumococcal double-glycine peptide is the competence-stimulating peptide (CSP) [18]. Other examples include the bacteriocininducing peptide (BIP) [19], the competence-induced bacteriocins (CibA and CibB) [20], peptides of the bacteriocin immunity region (BIR) [21], the virulence peptide 1 (VP1) [22], the biofilm-regulating peptide induced by competence (BriC) [23,24], peptides of the rtg locus [25], and LanA [22,26] (Table 1, S1 Table ). Furthermore, comparative genomic approaches have revealed additional double-glycine peptides that remain to be characterized [22,27].…”

“…However, not all transporters are promiscuous. Peptides of the rtg locus are not secreted by ComAB or BlpAB and instead are exported by another transporter, RtgAB [25]. Finally, BlpI, a peptide from BIR, can be secreted by all 3, albeit at different The conserved N-terminal leader of double-glycine peptides guides them for processing and export via C39-peptidase domain containing ABC transporters, ComAB, BlpAB, or RtgAB.…”