Contribution of the multiple Type I signal peptidases to the secretome of Listeria monocytogenes: Deciphering their specificity for secreted exoproteins by exoproteomic analysis

• SipZ improves the secretion efficiency for a subset of extracellular virulence factors.
• Multiple SPases I are functionally redundant for the majority of the Sec-secreted exoproteins.
• No differential recognition of Sec-secreted exoproteins by SPases I
• The concept of major and minor SPases are not absolute but relative.
• New insight in the secretome and physiology of L. monocytogenes

As commonly seen in monoderm bacteria, Listeria monocytogenes possesses multiple membrane-bound signal peptidases of Type I (SPases I) called SipX, SipY and SipZ. In order to decipher their respective contribution in an integrated and global view, the complement of the secretome corresponding to the exoproteome was resolved by two-dimensional gel electrophoresis (2-DE). This was performed for L. monocytogenes sipX−, sipY−, sipZ− single mutants, as well as for ΔsipXY and ΔsipYZ double mutants, and then compared to that of the wild type strain. Remarkably, the amounts of listeriolysin O (LLO), phosphatidylcholine phospholipase C (PlcB) and zinc metalloproteinase Mpl in the extracellular milieu were significantly decreased upon inactivation of SipZ. For the majority of the Sec-secreted exoproteins identified, protein secretion was not affected by the inactivation of one or two of the SPases I, supporting the concept that the three SPases I have overlapping specificities for the cleavage of the signal peptides. The current study reveals that the role of SipZ as the major SPase I of L. monocytogenes applies only to a small subset of the secreted exoproteins. Rather than absolute, the notion of major and minor SPases thus appears to be relative. In addition to new insight into bacterial physiology, this investigation of the contribution of the SPases I to the exoproteome of L. monocytogenes paves the way for further characterization of other complements of the secretome under various environmental conditions.Biological significanceL. monocytogenes encodes three orthologous signal peptidases of Type I (SPases I). SipZ improves the secretion efficiency for a subset of extracellular virulence factors. Multiple SPases I are functionally redundant for the majority of the Sec-secreted exoproteins of L. monocytogenes. The concepts of major and minor SPases are not absolute but relative.

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