Research in the Bishop lab has been focused on the outer membrane enzyme PagP, which transfers a palmitate chain from a phospholipid molecule to lipid A or endotoxin. The past 5 years have been extremely rewarding and confusing at the same time. A serendipitous discovery of a new metabolic pathway was revealed to us 5 years ago, but it took us this long to figure out what was actually going on. We had discovered that PagP has an additional enzymatic activity in the transfer of a palmitate chain to the polar headgroup of phosphatidylglycerol (PG) to produce palmitoy-PG (PPG). We now know that PPG emerges in the external leaflet of the outer membrane and then flips to the inner leaflet where it encounters a new active site in PagP. Yes, PagP is a bifunctional enzyme with distinct active sites located on either side of the outer membrane. The new periplasmic active site can hydrolyze PPG to generate a new phospholipid not described previously in bacteria bis(monoacylglycero)phosphate (BMP). BMP is an isomer of PG that can be easily mistaken for PG when identified by mass alone. BMP also co-migrates with cardiolipin in standard chromatography analyses. Only by using a cardiolipin-deficient mutant of E. coli and separating BMP from PG could we make sense of our observations.
Different aspects of this research is the result of the dedicated work of three talented PhD students Liset Maldonado-Alvarez, Charneal Dixon, and Sanchia Miller who all will all be defending their theses as the summer comes to a close. These studies mark the end of an exciting new chapter in the investigation of PagP structure-function relationships that we hope will inspire new students to take up the challenge of elucidating the functions of these new lipids in signal transduction processes.