Transbilayer distribution of phospholipids in bacteriophage membranes

We have previously demonstrated that the membranes of several bacteriophages contain more phosphatidylglycerol (PG) and less phosphatidylethanolamine (PE) than the host membrane from where they are derived. Here, we determined the transbilayer distribution of PG and PE in the membranes of bacteriophages PM2, PRD1, Bam35 and phi6 using selective modification of PG and PE in the outer membrane leaflet with sodium periodate or trinitrobenzene sulfonic acid, respectively. In phi6, the transbilayer distributions of PG, PE and cardiolipin could also be analyzed by selective hydrolysis of the lipids in the outer leaflet by phospholipase A₂. We used electrospray ionization mass-spectrometry to determine the transbilayer distribution of phospholipid classes and individual molecular species. In each bacteriophage, PG was enriched in the outer membrane leaflet and PE in the inner one (except for Bam35). Only modest differences in the transbilayer distribution between different molecular species were observed. The effective shape and charge of the phospholipid molecules and lipid-protein interactions are likely to be most important factors driving the asymmetric distribution of phospholipids in the phage membranes. The results of this first systematic study on the phospholipid distribution in bacteriophage membranes will be very helpful when interpreting the accumulating high-resolution data on these organisms.