9.B.157 The Cell Shape-determining MreBCD (MreBCD) Family
The minCD/mreBCD genes together comprise an operon (Levin et al. 1992).The MreB (also called FtsA) proteins of Escherichia coli, Bacillus subtilis and Caulobacter crescentus form actin-like cables lying beneath the cell surface (Soufo and Graumann 2003). These MreB proteins are homologous to Hsp70 chaparone proteins (TC# 1.A.33) and distantly related to exopolyphosphatases andPpx/GppA family phosphatases. The cables are required to guide longitudinal cell wall synthesis, and their absence leads to merodiploid spherical and inflated cells prone to cell lysis. In B. subtilis, E. coli and C. crescentus, the mreB gene is essential. E. coli cells depleted of mreBCD became spherical, enlarged and finally lyse. Depletion of each mre gene separately conferred similar gross changes in cell morphology and viability. Thus, the three proteins encoded by mreBCD are all essential and function in the same morphogenetic pathway.
Interestingly, the presence of a multicopy plasmid carrying the ftsQAZ genes suppressed the lethality of deletions in the mre operon (Kruse et al. 2005). The MreC and MreD proteins associate with the cell membrane, and MreC interacts with both MreB and MreD. Thus, the E. coli MreBCD complex forms an essential membrane-bound complex. While MreB has 0 TMSs, MreC has 1 N-terminal TMS, and MreD has 5 or 6 TMSs. It has been proposed that the membrane-associated MreBCD complex directs longitudinal cell wall synthesis in a process essential to maintain cell morphology (Kruse et al. 2005). The Bacillus halodurans MreD is similar in sequence to members of ABC sub-family 3.A.1.26 in TCDB, warranting inclusion of MreBCD in TCDB. No direct evidence implicates MreD in transport.