Determination of the Stability of the Noncovalent Phospholipid Transfer Protein-lipid Complex by Electrospray Time-of-flight Mass Spectrometry

Phosphatidylcholine transfer protein (PC-TP) containing different molecular species of PC and phosphatidylinositol transfer protein alpha (PI-TPalpha) containing either a PI, PC, or PG molecule were identified as intact complexes by nano-electrospray ionization time-of-flight mass spectrometry. The stability of these complexes in the gas phase was determined by elevating the cone voltage (cv) resulting in the appearance of the protein void of lipid. PC-TP containing a PC species carrying an sn-1 palmitoyl chain was less stable than PC-TP containing a PC species carrying an sn-1 stearoyl chain given that these complexes were dissociated for 50% at a cv of roughly 30 and 45 V, respectively. Different acyl chains on the sn-2 position did not lead to significant changes in stability of the complex. In the case of PI-TPalpha, the complexes containing PI and PG were dissociated for 50% at a cv of 100 V as compared to a cv of 40 V for the complex containing PC. We propose that this difference in stability is due to hydrogen bonds between the polar headgroup of PI and PG and the lipid-binding site of PI-TPalpha. This may explain why PI-TPalpha preferentially binds PI from a membrane interface.