Phospholipids that Plug the Pores of Cholesteryl Ester Transfer Protein Control Its Triglyceride Transfer

Cholesteryl ester transfer protein (CETP), a key drug target in cardiovascular disease, primarily regulates plasma circulating levels of lipids (within high, low, and very-low-density lipoproteins). In addition to its terminal openings, CETP has 2 additional openings, plugged by a phospholipid (PL) each. However, the mechanism, by which CETP moves lipids between lipoproteins, and roles of PL plugs in CETP function remain elusive. Further, small-molecule inhibitors targeting CETP tunnel displace PL during CETP inhibition. Here, using steered molecular dynamics simulations followed by in vitro mutagenesis, we show that CETP-bound PLs are indispensable in establishing the optimal architecture of CETP tunnel. PLs were critical in synchronizing domain movements of CETP while accelerating triglyceride traversal through the tunnel and their activity regulated through salt bridge interactions. Most notably, PLs bound within the CETP tunnel accelerated lipid movement through a novel "gliding" mechanism, in which a bound PL directly facilitates the movement of a second lipid species. Structural and functional analyses revealed that lipid traversal through the central tunnel of CETP was facilitated through hydrophobic-interaction-mediated diffusion. Further, conserved phenylalanine flaps regulated lipid movement by concerted opening and closing to prevent lipid backflow in the absence of an active motor. This study provides in-depth understanding of the mechanism of lipid exchange by CETP, guided and accentuated by its interaction with PLs. The conserved nature of these critical structural elements suggests that this mechanism may extend to other members of this family, expanding our understanding of lipid transport in this clinically important protein class.