Yeast phosphatidic acid phosphatase Pah1 hops and scoots along the membrane phospholipid bilayer

Phosphatidic acid (PA) phosphatase, encoded by PAH1 in the yeast Saccharomyces cerevisiae, catalyzes the Mg²⁺-dependent dephosphorylation of PA, producing diacylglycerol at the nuclear/endoplasmic reticulum (ER) membrane. This enzyme plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. As an interfacial enzyme, PA phosphatase interacts with the membrane surface, binds its substrate, and catalyzes its reaction. The Triton X-100/PA-mixed micellar system has been utilized to examine the activity and regulation of yeast PA phosphatase. This system, however, does not resemble the in vivo environment of the membrane phospholipid bilayer. We developed an assay system that mimics the nuclear/ER membrane to assess PA phosphatase activity. PA was incorporated into unilamellar phospholipid vesicles (liposomes) composed of the major nuclear/ER membrane phospholipids phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine. We optimized this system to support enzyme—liposome interactions and to afford activity that is greater than that obtained with the aforementioned detergent system. Activity was regulated by phospholipid composition, whereas the enzyme’s interaction with liposomes was insensitive to composition. Greater activity was attained with large (> 100 nm) versus small (50 nm) vesicles. The fatty-acyl moiety of PA had no effect on this activity. PA phosphatase activity was dependent on the bulk (hopping mode) and surface (scooting mode) concentrations of PA suggesting a mechanism by which the enzyme operates along the nuclear/ER membrane in vivo.