Regulated Cleavage of Glycan Strands by the Murein Hydrolase SagB in Staphylococcus aureus Involves a Direct Interaction with LyrA (SpdC)

LyrA (SpdC), a homologue of eukaryotic CAAX proteases that act on prenylated substrates, has been implicated in the assembly of several pathways of the envelope of Staphylococcus aureus. We described earlier the lysostaphin resistance (Lyr) and staphylococcal protein A display (Spd) phenotypes associated with loss of the lyrA (spdC) gene. However, a direct contribution to the assembly of pentaglycine cross bridges, the target of lysostaphin cleavage in S. aureus peptidoglycan or of staphylococcal protein A attachment to peptidoglycan, could not be attributed directly to LyrA (SpdC). These two processes are catalyzed by the Fem factors and Sortase A, respectively. To gain insight into the function of LyrA (SpdC), here we use affinity chromatography and LC-MS/MS analysis and report that LyrA interacts with SagB. SagB cleaves glycan strands of peptidoglycan to achieve physiological length. Similar to sagB peptidoglycan, lyrA peptidoglycan contains extended glycan strands. Purified lyrA peptidoglycan can still be cleaved to physiological length by SagB in vitro. LyrA does not modify or cleave peptidoglycan, and it also does not modify or stabilize SagB. The membrane-bound domain of LyrA is sufficient to support SagB activity, but predicted CAAX enzyme catalytic residues in this domain are dispensable. We speculate that LyrA exerts its effect on bacterial prenyl substrates, specifically undecaprenol-bound peptidoglycan substrates of SagB, to help control glycan length. Such an activity also explains the Lyr and Spd phenotypes observed earlier. IMPORTANCE Peptidoglycan is assembled on the trans side of the plasma membrane from lipid II precursors into glycan chains that are cross-linked at stem peptides. In S. aureus, SagB, a membrane-associated N-acetylglucosaminidase, cleaves polymerized glycan chains to their physiological length. Deletion of sagB is associated with longer glycan strands in peptidoglycan, altered protein trafficking and secretion in the envelope, and aberrant excretion of cytosolic proteins. It is not clear whether SagB, with its single transmembrane segment, serves as the molecular ruler of glycan chains or whether other factors modulate its activity. Here, we show that LyrA (SpdC), a protein of the CAAX type II prenyl endopeptidase family, modulates SagB activity via interaction though its transmembrane domain.

Automated summary

LyrA modulates the length of glycan chains in Staphylococcus aureus peptidoglycan by interacting with SagB, without directly modifying or cleaving the peptidoglycan. This activity also explains the Lyr and Spd phenotypes observed earlier.