A hydrophobic cluster at the surface of the human plasma phospholipid transfer protein is critical for activity on high density lipoproteins

The plasma phospholipid transfer protein (PLTP) belongs to the lipid transf er/lipopolysaccharide binding protein (LT/LBP) family, together with the ch olesteryl ester transfer protein, the lipopolysaccharide binding protein (L BP) and the bactericidal permeability increasing protein (BPI). In the pres ent study, we used the crystallographic data available for BPI to build a t hree-dimensional model for PLTP. Multiple sequence alignment suggested that , in PLTP, a cluster of hydrophobic residues substitutes for a cluster of p ositively charged residues found on the surface of LBP and BPI, which is cr itical for interaction with Lipopolysaccharides. According to the PLTP mode l, these hydrophobic residues are situated on an exposed hydrophobic patch at the N-terminal tip of the molecule. To assess the role of this hydrophob ic cluster for the functional activity of PLTP, single point alanine mutant s were engineered. Phospholipid transfer from liposomes to high density lip oprotein (HDL) by the W91A, F92A, and F93A PLTP mutants was drastically red uced, whereas their transfer activity toward very low density lipoprotein a nd low density lipoprotein did not change, The HDL size conversion activity of the mutants was reduced to the same extent as the PLTP transfer activit y toward HDL. Based on these results, we propose that a functional solvent- exposed hydrophobic cluster in the PLTP molecule specifically contributes t o the PLTP transfer activity on HDL substrates,