Biosynthesis of peptide signals in gram-positive bacteria

Abstract

Gram-positive bacteria coordinate social behavior by sensing the extracellular level of peptide signals. These signals are biosynthesized through divergent pathways and some possess unusual functional chemistry as a result of posttranslational modifications. In this chapter, the biosynthetic pathways of Bacillus intracellular signaling peptides, Enterococcus pheromones, Bacillus subtilis competence pheromones, and cyclic peptide signals from Staphylococcus and other bacteria are covered. With the increasing prevalence of the cyclic peptide signals in diverse Gram-positive bacteria, a focus on this biosynthetic mechanism and variations on the theme are discussed. Due to the importance of peptide systems in pathogenesis, there is emerging interest in quorum-quenching approaches for therapeutic intervention. The quenching strategies that have successfully blocked signal biosynthesis are also covered. As peptide signaling systems continue to be discovered, there is a growing need to understand the details of these communication mechanisms. This information will provide insight on how Gram-positives coordinate cellular events and aid strategies to target these pathways for infection treatments.

Figure 1

Examples of quorum-sensing signals produced by Gram-positive bacteria. (A) Unmodified linear peptide signals produced by Bacillus and Streptococcus. (B) Enterococcus plasmid pheromones and their corresponding inhibitors. (C) The cyclic autoinducing peptides produced by many Gram positives. (D) The ComX competence pheromone of B. subtilis 168. The tryptophan residue is farnesylated through an unusual posttranslational modification.

Figure 2

Comparison of peptide signal biosynthetic pathways in Gram positives. (A) The biosynthesis of intracellular acting peptides in Bacillus. The active region is usually on the C-terminus of the precursor peptide, and the leader is a standard signal sequence (SS). Following Sec secretion, the precursor is processed outside the cell by extracellular proteases. (B) The biosynthesis of Enterococcus pheromones. The precursor has a Type II signal sequence and a long lipoprotein C-terminal tail (not drawn to scale). Type II signal peptidase (SpII) removes the lipoprotein tail, and an integral membrane peptidase (Eep) removes the N-terminal leader (SS-II). In some cases, extracellular processing also occurs. (C) The biosynthesis of cyclic peptide signals. The precursor has an amphipathic leader and a charged C-terminal tail. The tail is removed by an AgrB-like membrane endopeptidase, and the leader is removed by Type I signal peptidase (SpI). AgrB is proposed to also catalyze the ring formation and export of the AIP signal.

Figure 3

CSF biosynthesis in B. subtilis. The precursor of the CSF signal is ribosomally synthesized from the phrC gene. Following synthesis, the signal sequence directs the section through the Sec system, and Type I signal peptidase removes the leader. Outside the cell, the propeptide is processed to the active form of CSF through the cleavage activities of the extracelluar serine proteases subtilisin, Epr, and Vpr. Once active CSF is generated, the signal can be reimported through an oligopeptide permease (Opp), and CSF can exert its regulatory function inside the cell

Figure 4

Schematic of AIP biosynthesis in S. aureus. Step 1: the AgrD N-terminal amphipathic helix associates with the cytoplasmic membrane. Step 2: the endopeptidase activity of AgrB removes the C-terminal tail of AgrD. Step 3: the remaining AgrD peptide fragment (N-terminal and AIP regions) is covalently bound to AgrB as a linked thioester at cysteine residue C84. Nucleophilic attack of the AgrD cysteine residue (C28) can displace the peptide and form the thiolactone ring through thioester exchange. Step 4: the N-terminal leader and thiolactone is transported to the outer face of the membrane. Step 5: Type I signal peptidase (SpsB) removes the leader and releases AIP into the extracellular environment. This figure was originally published in the Journal of Biological Chemistry.

Figure 5

Strategies to quench peptide communication mechanisms. A schematic of the S. aureus agr system is shown as an example. A common approach for interrupting peptide quorum sensing is through receptor antagonists, and there are many examples of successful inhibitors that have been identified through synthesis and screening approaches. Recently, an antibody sequestering strategy against AIP was shown to be successful in quenching the S. aureus agr system in vitro and in vivo. For interrupting signal biosynthesis, several successful attempts have been reported, the most notable being against the E. faecalis fsr system.