Anchor structure of staphylococcal surface proteins - A branched peptide that links the carboxyl terminus of proteins to the cell wall

Surface proteins of Staphylococcus aureus are anchored to the cell wall by a mechanism requiring a COOH-terminal sorting signal, Previous work demonstrated that the sorting signal is cleaved at the conserved LPXTG motif and that the carboxyl of threonine (T) is linked to the staphylococcal cell wall. By employing different cell wall lytic enzymes, surface proteins were released from the staphylococcal peptidoglycan and their COOH-terminal anchor structure was revealed by a combination of mass spectrometry and chemical analysis, The results demonstrate that surface proteins are linked to a branched peptide (NH2-Ala-gamma-Gln-Lys-(NH2-Gly(5))-Ala-COOH) by an amide bond be tween the carboxyl of threonine and the amino of the pentaglycine cross-bridge that is attached to the epsilon-amino of lysyl, This branched anchor peptide is amide-linked to the carboxyl of N-acetylmuramic acid, thereby tethering the COOH-terminal end of surface proteins to the staphylococcal peptidoglycan.