Abstract
Surface proteins of Staphylococcus aureus are secreted across septal membranes for assembly into the bacterial cross-wall. This localized secretion requires the YSIRK/GXXS motif signal peptide, however the mechanisms supporting precursor trafficking are not known. We show here that the signal peptide of staphylococcal protein A (SpA) is cleaved at the YSIRK/GXXS motif. A SpA signal peptide mutant defective for YSIRK/GXXS cleavage is also impaired for septal secretion and co-purifies with SecA, SecDF and LtaS. SecA depletion blocks precursor targeting to septal membranes, whereas deletion of secDF diminishes SpA secretion into the cross-wall. Depletion of LtaS blocks lipoteichoic acid synthesis and abolishes SpA precursor trafficking to septal membranes. We propose a model whereby SecA directs SpA precursors to lipoteichoic acid-rich septal membranes for YSIRK/GXXS motif cleavage and secretion into the cross-wall.
Highlights
Surface proteins of Staphylococcus aureus and other gram-positive cocci enter the secretory pathway with their N-terminal signal peptides (DeDent et al, 2008)
Common features of staphylococci and streptococci are their spherical or ovoid cell shapes and cell wall synthesis programs at septal membranes; in staphylococci this compartment is designated as the cross-wall (Giesbrecht et al, 1976; Touhami et al, 2004; Monteiro et al, 2015)
Earlier work demonstrated that the YSIRK/GXXS motif of the staphylococcal protein A (SpA) precursor is dispensable for sortase-catalyzed cell wall anchoring (Bae and Schneewind, 2003)
Summary
Surface proteins of Staphylococcus aureus and other gram-positive cocci enter the secretory pathway with their N-terminal signal peptides (DeDent et al, 2008). Once translocated across the membrane, surface proteins are covalently linked to cell wall peptidoglycan via sortase A-catalyzed cleavage at the LPXTG motif of C-terminal sorting signals (Schneewind et al, 1992; Schneewind et al, 1995; Mazmanian et al, 1999). But not all surface proteins are secreted at septal membranes and incorporated into cross-wall peptidoglycan (Cole and Hahn, 1962; Carlsson et al, 2006; DeDent et al, 2008). Cross-wall trafficking of surface proteins requires a signal peptide with YSIRK/GXXS motif (Carlsson et al, 2006; DeDent et al, 2008). The YSIRK/GXXS motif is positioned N-terminal of the hydrophobic core, common to all signal peptide precursors traveling the Sec pathway (Emr et al, 1978; Emr et al, 1981; von Heijne, 1986)
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