Substrate specificity and membrane topology of Escherichia coli PgpB, an undecaprenyl pyrophosphate phosphatase

The synthesis of the lipid carrier undecaprenyl phosphate (C-55-P) requires the dephosphorylation of its precursor, undecaprenyl pyrophosphate (C-55-PP). The latter lipid is synthesized de novo in the cytosol and is also regenerated after its release from the C-55-PP-linked glycans in the periplasm. In Escherichia coli the dephosphorylation of C-55-PP was shown to involve four integral membrane proteins, BacA, and three members of the type 2 phosphatidic acid phosphatase family, PgpB, YbjG, and YeiU. Here, the PgpB protein was purified to homogeneity, and its phosphatase activity was examined. This enzyme was shown to catalyze the dephosphorylation of C-55-PP with a relatively low efficiency compared with diacylglycerol pyrophosphate and farnesyl pyrophosphate (C-15-PP) lipid substrates. However, the in vitro C-55-PP phosphatase activity of PgpB was specifically enhanced by different phospholipids. We hypothesize that the phospholipids are important determinants to ensure proper conformation of the atypical long axis C-55 carrier lipid in membranes. Furthermore, a topological analysis demonstrated that PgpB contains six transmembrane segments, a large periplasmic loop, and the type 2 phosphatidic acid phosphatase signature residues at a periplasmic location.