Sortase: the surface protein anchoring transpeptidase and the LPXTG motif

Abstract

Although it is clear that the correct gene has been isolated, an attempt to repair the thermosensitivity of the sortase mutant by complementation has met with minimal success, suggesting that sortase might not be essential after all and that the temperature sensitivity was owing to a coincidental mutation in a second, essential gene. This has recently been corroborated by the finding that a sortase knockout is viable (O. Schneewind, pers. commun.). Although widely predicted on the presumption of essentiality to be an attractive target for the identification of novel antimicrobials, the non-essential nature of sortase has required some rethinking – presently, virulence tests of the sortase knockout are underway, with the idea that an anti-sortase agent could block virulence in a wide spectrum of Gram-positive bacteria, even if it did not block growth.Interestingly, in the sortase mutant, as with protein mutants lacking the LPXTG motif, the proteins that would normally be attached to the cell wall peptidoglycan do not associate with the cell wall, confirming that the carboxy- terminal membrane anchor is not sufficient, nor do they accumulate, either extra- or intracellularly (O. Schneewind, pers. commun.). Perhaps the accumulation of membrane-bound surface protein intermediates that cannot be sorted or secreted is toxic to the cell – as was originally found for Escherichia coli β galactosidase fused to a secretion signal17xSuppressor mutations that restore export of a protein with a defective signal sequence. Emr, S.D. et al. Cell. 1981; 23: 79–88Abstract | Full Text PDF | PubMed | Scopus (154)See all References, 18xE. coli mutant pleiotropically defective in the export of secreted proteins. Oliver, D.B. and Beckwith, J. Cell. 1981; 25: 765–772Abstract | Full Text PDF | PubMed | Scopus (203)See all References. Accordingly, there is probably an auto-regulatory system governing the synthesis and fate of the surface proteins, preventing their accumulation when sorting is defective.In conclusion, the elegant recent work of Schneewind and co- workers has provided final confirmation for the idea that surface proteins in Gram-positive bacteria are covalently anchored to the cell wall. Although this has been quite clear for nearly 30 years, nagging doubts remained and these must finally be laid to rest by the isolation and analysis of sortase.