This Staphylococcus aureus enzyme catalyses the successive transfer of two Gly moieties from charged tRNAs to MurNAc-L-Ala-D-isoglutaminyl-L-Lys-(N6-tri-Gly)-D-Ala-D-Ala-diphosphoundecaprenyl-GlcNAc, attaching them to the three Gly molecules that were previously attached to the N6 of the L-Lys at position 3 of the pentapeptide by EC 126.96.36.199 (lipid II:glycine glycyltransferase) and EC 188.8.131.52 (N-acetylmuramoyl-L-alanyl-D-glutamyl-L-lysyl-(N6-glycyl)-D-alanyl-D-alanine-diphosphoundecaprenyl-N-acetylglucosamine:glycine glycyltransferase). This is the last step in the synthesis of the pentaglycine interpeptide bridge that is used in this organism for the crosslinking of different glycan strands to each other.
FemB catalyzes the third step in the synthesis of the pentaglycine interpeptide bridge crosslinking different glycan strands in Staphylococcus aureus. FemX adds the first glycine residue to MurNAc-L-Ala-D-Glu-(N6-Gly)L-Lys-D-Ala-D-Ala-diphosphoundecaprenyl-M-acetylglucosamine, i.e. lipid II. Addition of glycine residues 2, 3 and glycine residues 4, 5 is catalyzed by enzymes FemA and FemB, respectively. None of the FemABX enzymes requires the presence of one or two of the other Fem proteins for activity, rather, bridge formation is delayed in an in vitro system when all 3 enzymes are present
FemB is involved in the addition of exclusively glycine residues 4 and 5 to the staphylococcal interpeptide bridge. femA mutants leading to truncated proteins still produce intact FemB while exhibiting a phenotype identical to femAB double mutants, such as same muropeptide pattern. FemA is essential for the addition of glycine residues 2 and 3 only to the staphylococcal interpeptide bridge, and FemB cannot substitute for FemA
surface protein is linked to tri- and monoglycyl cross-bridges of peptidoglycan isolated from femB and femA mutant staphylococci, respectively. Peptidoglycan analysis of a femAB mutant strain reveals the presence of pentaglycyl, tetraglycyl-monoseryl, and monoglycyl as well as small amounts of triglycyl cross-bridges. Analysis of anchor peptides shows that surface proteins are mostly linked to tetraglycylmonoseryl as well as pentaglycyl. The sortase activity of Staphylococcus aureus prefers cross-bridges containing five residues, but altered cell-wall cross-bridges can be linked to the COOH-terminal end of surface proteins
in strains carrying mutations of FemA, femAB, or the femAX genes, the sorting reaction of surface proteins is significantly slowed. Strains carrying mutations in the fem genes display a decreased rate of surface protein precursor cleavage as compared with the wildtype strains, suggesting that the altered cross-bridges slow the anchoring of surface proteins
the lysostaphin immunity factor Lif is able to complement lack of FemB by inserting serine for glycine in the side chain. Methicillin resistance, which depends on functional FemA and FemB, is not complemented by Lif suggesting that serine-substituted side-chains are a lesser substrate for penicillin-binding protein PBP2' in methicillin resistance