Myxosortases process MYXO-CTERM and other bacterial C-terminal protein-sorting signals that have invariant Cys residues

The LPXTG protein-sorting signal, found in surface proteins of various Gram-positive pathogens, was the founding member of a growing panel of prokaryotic small C-terminal sorting domains. Sortase A (SrtA) cleaves LPXTG, exosortases (XrtA and XrtB) cleave the PEP-CTERM sorting signal, archaeosortase A (ArtA) cleaves PGF-CTERM, and rhombosortase (RrtA) cleaves GlyGly-CTERM domains. Three sorting signal domains without previously known processing proteases are the MYXO-CTERM, JDVT-CTERM, and SYNERG-CTERM domains. These exhibit the standard tripartite architecture of short signature motif, then a hydrophobic transmembrane segment, then an Arg-rich cluster. Each has an invariant cysteine in its signature motif. Here, we show computational evidence that these three Cys-containing sorting signals are processed by corresponding subfamilies of glutamic-type intramembrane proteases, related to type II CAAX-processing proteases found in eukaryotes. We name these sorting enzymes generally as myxosortases, and identify MXAN_2755 from Myxococcus xanthus as MrtX (myxosortase X). Additional myxosortases families MrtC and MrtP have radically different N-terminal domains, suggesting most myxosortases act as bifunctional enzymes. Myxosortase-like processing enzymes are identified also for the JDVT-CTERM (MrtJ) and SYNERG-CTERM (MrtS). This work establishes a major new family of protein-sorting housekeeping enzymes for the surface attachment of proteins on bacterial outer membranes.