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.
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The MreBCD complex which forms an essential membrane associated complex. MreD is a 5 TMS protein resembling members of TC family with sequence similarity to the integral membrane transport protein listed under TC# 3.A.1.26.10.
Firmicutes
MreBCD of Bacillus subtilis
MreB, 337 aas, (Q01465)
MreC, 290 aas, (Q01466)
MreD, 172 aas, (Q01467)
The MreBCD complex.
Firmicutes
MreBCD of Bacillus halodurans
MreB, 346 aas (Q9K8H5)
MreC, 293 aas (Q9K8H6)
MreD, 174 aas (Q9K8H7)
Cell shape determining complex, MreBCD (347, 367 and 162 aas, respectively) (Kruse et al. 2005). MreB is homologous to the chaperone protein Hsc70 (TC# 1.A.33.1.1) and the HscA homologues presented under TC#s 9.B.157.1.1 - 3.
Proteobacteria
MreBCD of E. coli
MreBCD homologues
Spirochaetes
MreBCD homologues of Leptospira interrogans
MreB, 409 aas (Q8F2L2)
MreC, 377 aas (Q8F2L4)
MreD, 170 aas (Q8F2L5)
MreBCD cell shape-determining complex: Three proteins involved in Streptomyces coeicolor spore cell wall synthesis to form the thickened septum (relative to the vegegative cell wall). MreB is 342 aas long with possibly 1 TMS at about residue 180, a second protein, MreC, which is 366 residues long with possibly 3 TMSs, one N-terminal, one at about residue 160, and one at about residue 280, and MreD, which is 223 aas long and has 5 TMSs. This system has been characterized (Sigle et al. 2015; Vollmer et al. 2019). MreB gives a TC BLAST score of e-128; MreC gives a TC BLAST score of e-10, and MreD give a TC BLAST score of 0.0001 all with 9.B.157.1.4, showing that all 3 protein constituents are homology with other members of the family, but with greatly differing degrees of sequence similarities. MreD with 5 TMSs gives a TC BLAST score of 0.0004 with T# 3.A.1.28.2, suggesting a relationship with an ABC-type membrane transport protein.
MreBCD of Streptomyces coeicolor