Same, yet different: characterization of bacterial S-type and R-type phospholipid N-methyltransferases

Phosphatidylcholine (PC) was long thought to be a solely eukaryotic phospholipid. Its presence was only recently discovered in several prokaryotic organisms, where the amount of PC ranges from a few percent of total membrane lipids in Xanthomonas campestris to about 70% in Acetobacter aceti. Interestingly, many of the bacterial species with PC in their membrane have been demonstrated to engage in either symbiotic or pathogenic microbe-host interactions.

One of the most common PC biosynthesis pathways in bacteria is a threefold S-adenosylmethionine (SAM)-dependent methylation of phosphatidylethanolamine (PE) to PC catalyzed by phospholipid N-methyltransferases (Pmts). Pmts are classified based on their sequence in two groups: Sinorhizobium (S) and Rhodobacter (R). Regardless of their type, Pmt enzymes display different substrate preferences. For instance, the S-type Agrobacterium tumefaciens PmtA catalyzes all three methylations of PE, while another S-type Thermobifida fusca Pmt only executes the first two methylation steps. So far, S-type PmtA from A. tumefaciens is the best-characterized Pmt with extensively studied membrane binding sites, SAM-binding motif and order of reaction steps. However, the molecular details of substrate selectivity and the nature of the active site and the catalytic mechanism remain poorly understood. Furthermore, very little is known about R-type Pmts in general.

The main goal of this project is a comparative analysis of S-type and R-type Pmts. To identify conserved and unique enzyme regions responsible for the substrate selectivity and enzymatic activity, we employ a mix of biochemical and computational research, as well as site-directed mutagenesis experiments.