The roles of the carboxyl-terminal regions of phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) in lipid transfer.

Abstract

In human plasma, there are two lipid transfer proteins that play major roles in lipoprotein metabolism and reverse cholesterol transport. Cholesterol ester transfer protein (CETP) transfers neutral lipids, and is responsible for all of the plasma cholesteryl ester (CE) and triglyceride (TG) transfer activity. Both CETP and phosopholipid transfer protein (PLTP) can facilitate the exchange of phospholipid (PL) among lipoproteins, but only PLTP mediates net mass PL transfer. Residues at the carboxy termini of CETP and PUP have been shown to be essential for protein function. In order to examine the roles of the C-termini of CETP and PUP in determining lipid specificity, we have expressed and characterized chimeric proteins in which the C-termini of PUP and CETP are exchanged. The chimeras CETP1--460/PLTP 445--476 (CP), CETP1--460/PLTP445--476 -mycHIS (CP-HIS), and PLTP1--444/CETP461--476 (PC) were secreted from COS-7 cells, indicating that the overall fold of the proteins was not disturbed. The proteins CP, CPHIS, and PC each had less than 10% of the CE and TG transfer activities of CETP. The proteins CP and CPHIS each had less than 10% of the PLTP-specific PL transfer activity of PUP. These results suggest that the carboxy-termini of CETP and PLTP act in concert with other parts of the molecule to facilitate lipid transfer. Another goal of this research was to produce and characterize a panel of monoclonal antibodies (mAbs) to human PLTP. Attempts to produce mAbs in mice using protein immunization were unsuccessful, but PLTP-specific antibodies were detected in serum from mice immunized with plasmid DNA encoding the human PLTP sequence.