Dimeric structure of the cell shape protein MreC and its functional implications

Abstract

SummaryThe bacterial actin homologue MreB forms helical filaments in the cytoplasm of rod-shaped bacteria where it helps maintain the shape of the cell. MreB is co-transcribed with mreC that encodes a bitopic membrane protein with a major periplasmic domain. Like MreB, MreC is localized in a helical pattern and might be involved in the spatial organization of the peptidoglycan synthesis machinery. Here, we present the structure of the major, periplasmic part of MreC from Listeria monocytogenes at 2.5 Å resolution. MreC forms a dimer through an intimate contact along an N-terminal a-helix that connects the transmembrane region with two C-terminal b-domains. The translational relationship between the molecules enables, in principle, filament formation. One of the b-domains shows structural similarity to the chymotrypsin family of proteins and possesses a highly conserved Thr Ser dipeptide. Unexpectedly, mutagenesis studies show that the dipeptide is dispensable for maintaining cell shape and viability in both Escherichia coli and Bacillus subtilis. Bacterial two-hybrid experiments reveal that MreC interacts with highmolecular-weight penicillin-binding proteins (PBPs), rather than with low-molecular-weight endo-and carboxypeptidases, indicating that MreC might act as a scaffold to which the murein synthases are recruited in order to spatially organize the synthesis of new cell wall material. Deletion analyses indicate which domains of B. subtilis MreC are required for interaction with MreD as well as with the PBPs.

Coarse-grained simulations of bacterial cell wall growth reveal that local coordination alone can be sufficient to maintain rod shape

“…In our model, direct transfer of PG precursors from the cytoplasm to transglycosylases facilitates processivity and rapid re-initiation. Consistent with these ideas, the putative bridging proteins MreC, MreD, RodA, and RodZ have been shown to be essential for rod shape maintenance (21,74,(77)(78)(79)(80)(81)(82)(83) and interact with both cytoplasmic (84) and periplasmic enzymes (85,86). Also, treatment with A22 to depolymerize MreB dramatically reduces PG synthesis in the periplasm (87) and results in shorter glycan strands (88).…”

The Requirement for Pneumococcal MreC and MreD Is Relieved by Inactivation of the Gene Encoding PBP1a

“…MreC and MreD from S. pneumoniae have several remarkable features compared to their homologues in rod-shaped bacterial species (3,15,27,31,32). Like MreC in E. coli and B. subtilis (27,62), pneumococcal S. pneumoniae MreC (MreC Spn ) is an abundant membrane-bound protein of about 8,500 dimers per cell (see Fig. S5 in the supplemental material).…”

“…Fractionation and two-hybrid analyses demonstrated that E. coli and B. subtilis MreC are in the cell membrane, where they interact with MreD (28,62). MreC has been shown to interact with multiple PBPs in C. crescentus and B. subtilis (13,62). In E. coli, C. crescentus, and B. subtilis, MreC localizes helically along the cylindrical walls of cells (15,31,67).…”

“…The depletion of MreC or MreD in rod-shaped bacteria leads to the formation of spherical cells and eventual lysis (3,27,31). Fractionation and two-hybrid analyses demonstrated that E. coli and B. subtilis MreC are in the cell membrane, where they interact with MreD (28,62). MreC has been shown to interact with multiple PBPs in C. crescentus and B. subtilis (13,62).…”

“…The exact functions of MreC and MreD remain unknown. MreC consists of three domains: a cytoplasmic amino-terminal tail, a membrane-spanning domain, and an extracytoplasmic coiled-coil domain required for self dimerization (62). MreD is an integral membrane protein with five membrane-spanning domains (27,31).…”