The C-terminus of phosphatidylinositol transfer protein modulates membrane interactions and transfer activity but not phospholipid binding.

Rat phosphatidylinositol transfer protein (PITP) is a 32 kDa protein containing 271 amino acids. It is involved in a number of cell functions including secretion and cell signaling. To further characterize structure/activity relationships of PITP, two C-terminal truncated derivatives, PITP(1-259) and PITP(1-253), were produced in Escherichia coli and purified to homogeneity. PITP(1-259) had transfer activity equal to 30-40% to that of native PITP in transfer of either phosphatidylcholine (PC) or phosphatidylinositol (PI) when transfer was measured using 95/5 mol% PC/PI donor and acceptor vesicles; PITP(1-253) had only slight transfer activity, even under the most favorable assay conditions. Thus, amino acids 254-258 are critical for transfer activity. The transfer activity of PITP(1-259) was strongly dependent on the composition of the donor and acceptor vesicles. With 100 mol% PC donor and acceptor vesicles, PITP(1-259) transfer activity ranged from 70 to 100% to that of PITP. The presence of 2 mol% phosphatidic acid (PA) in either donor or acceptor vesicles reduced transfer activity to between 10 and 20% that of full-length PITP under the same conditions. If both donor and acceptor contained 2% PA, PITP(1-259) was essentially inactive, though the activity of PITP was not affected significantly under these conditions. PITP(1-253) and PITP(1-259) bind much more avidly to vesicles than does PITP, and this enhanced binding reflects increased electrostatic interactions. Thus, the C-terminal residues modulate the affinity of PITP for vesicles and the efficiency of phospholipid transfer.