9.B.3 The Putative Bacterial Murein Precursor Exporter (MPE) Family

Members of the MPE family are found in a large variety of Gram-negative and Gram-positive bacteria including E. coli, Haemophilus influenzae, Helicobacter pylori, Aquifex aeolicus, Bacillus subtilis, Streptococcus pneumoniae and Streptomyces coelicolor. They consist of 370-420 amino acyl residues with 9 (RodA) or 10 (FtsW) putative transmembrane α-helical spanners. Experimental evidence for a 10 TMS model has been reported for FtsW of Streptococcus pneumoniae (Gérard et al., 2002). The S. pneumoniae protein has both its N- and C-termini in the cytoplasm, a large (~60 residue) cytoplasmic domain between TMSs 4 and 5, and a large (~80 residue) extracytoplasmic loop between TMSs 7 and 8.

The best characterized members of the family are the FtsW cell division protein, the RodA rod shape determining protein (both of E. coli) and the SpoVE protein of B. subtilis. They have been suggested to function in the translocation (export) of lipid-linked murein precursors such as NAG-NAM-pentapeptide pyrophosphoryl undecaprenol (lipid II). They interact with murein synthases as well as two transpeptidases (PBP2 and PBP3). In Gram-negative bacteria the ftsW gene is physically linked to murG which is responsible for the final cytoplasmic step in the synthesis of lipid II before it is flipped to the periplasmic side of the membrane. They may therefore be part of a tunneling device directing the flow of murein precursors to the membrane enzymes that insert the precursors into the preexisting murein sacculus.

The putative reaction catalyzed by the proteins of the MPE family is:

Lipid-linked murein precursor (in) → Lipid-linked murein precursor (out).