Gram-negative lipoprotein-peptidoglycan tether remodeling

A reusable Gram-negative bacterial module for reversible covalent attachment of an outer-membrane lipoprotein to peptidoglycan. An ErfK-family L,D-transpeptidase transfers a tetrapeptide-stem donor onto the C terminus of a Braun-lipoprotein-like substrate, and a YafK/LdtF-family cysteine hydrolase can release the lipoprotein by cleaving the resulting amide bond. These reactions remodel the connection between the outer membrane and sacculus; they do not represent peptidoglycan glycan polymerization or canonical 4-3 peptide crosslinking by penicillin-binding proteins.

The two PSEPK proteins are assigned by exact PANTHER subfamily correspondence to functionally distinguished Pseudomonas aeruginosa enzymes. The anchoring activity of the PP_2320-related enzyme was demonstrated directly, whereas hydrolysis by PA14 LdtPae3 was assigned tentatively and remains an orthology-based prediction for PP_1451. The PA14 experimental accessions are resolved by locus: PA14_27180 is A0A0H2ZCT9 and PA14_15840 is A0A0H2ZF55. The hydrolysis family is independently anchored by experimentally characterized E. coli DpaA/LdtF (P0AA99), which shares PTHR36699:SF1 with PP_1451. PP_2320 is near oprI in KT2440 with PP_2321 intervening; this proximity is corroborative context, not evidence for a defined operon. GO:0071972 is asserted only for tether formation; no GO molecular-function term currently names hydrolysis of the peptidoglycan-protein tether, so that leaf retains a precise free-text function without an invented ontology identifier. Covalent lipoprotein-peptidoglycan tethering is not universal among Gram-negative bacteria, and the exact lipoprotein acceptor chemistry can differ between the enterobacterial Lpp and pseudomonad OprI systems; the module therefore describes reaction roles without imposing the E. coli Lpp C-terminal-lysine mechanism on every taxon.