MUTANT RAT PHOSPHATIDYLINOSITOL PHOSPHATIDYLCHOLINE TRANSFER PROTEINS SPECIFICALLY DEFECTIVE IN PHOSPHATIDYLINOSITOL TRANSFER - IMPLICATIONS FOR THE REGULATION OF PHOSPHOLIPID TRANSFER ACTIVITY

The mammalian phosphatidylinositol/phosphatidylcholine transfer proteins (PI-TPs) catalyze exchange of phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane bilayers in vitro, We find that Ser-25, Thr-59, Pro-78, and Glu-248 make up a set of rat (r) PI-TP residues, substitution of which effected a dramatic reduction in the relative specific activity for PI transfer activity without significant effect on PC transfer activity, Thr-59 was of particular interest as it is a conserved residue in a highly conserved consensus protein kinase C phosphorylation motif in metazoan PI-TPs, Replacement of Thr-59 with Ser, Gin, Val, Ile, Asn, Asp, or Glu effectively abolished PI transfer capability but was essentially silent with respect to PC transfer activity, These findings identify rPI-TP residues that likely cooperate to form a PI head-group binding/recognition site or that lie adjacent to such a site, Finally, the selective sensitivity of the PI transfer activity of rPI-TP to alteration of Thr-59 suggests a mechanism for in vivo regulation of rPI-TP activity.