More than just lysins: peptidoglycan hydrolases tailor the cell wall

Abstract

Enzymes that degrade the peptidoglycan (PG) cell wall layer called PG hydrolases or autolysins are often thought of as destructive forces. Phages employ them to lyse their host for the release of virion particles and some bacteria secrete them to eliminate (lyse) their competition. However, bacteria also harness the activity of PG hydrolases for important aspects of growth, division, and development. Of course, using PG hydrolases in this capacity requires that they be tightly regulated. While this has been appreciated for some time, we are only just beginning to understand the mechanisms governing the activities of these 'tailoring' enzymes. This review will focus on recent advances in this area with an emphasis on the regulation of PG hydrolases involved in cell division.

Figure 1

Overview of PG structure and modes of division. (a) Diagram of the chemical structure of PG: M, N-acetylmuramic acid; G, N-acetylglucosamine. Colored dots represent the attached peptides. The PG structure continues in all directions to envelop the cell (green arrows). Red arrows indicate cleavage locations for PG hydrolases: 1) N-acetylglucosaminidases, 2) N-acetylmuramidases and lytic transglycosylases, 3) endopeptidases, and 4) amidases. Classic designations for bacterial envelope types are shown below the PG structure. Gram-negative bacteria have a thin PG layer surrounded by an outer membrane. Gram-positive bacteria have a single membrane surrounded by a thick PG layer. (b) Basic modes of cell division. In bacteria that construct a flat septum, PG splitting is initiated some time after the completion of membrane fission. In bacteria that constrict, splitting is coordinated with membrane invagination. (c) Close-up diagram of the coordinated constriction of cell envelope layers during division of E. coli. Septal PG splitting is coordinated with inner membrane (IM) constriction and allows constriction of the outer membrane (OM). Proteins involved in the septal PG splitting process are indicated in the diagram. The FtsEX ABC system controls the activity of the EnvC protein, which serves as an activator of the PG amidases AmiA and AmiB. See text for details.