Abstract
Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal β-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.
Highlights
Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues
VLDLs can exchange their containing TGs with HDL cholesteryl ester (CE) mediated by CE transfer protein (CETP) via a tunnel mechanism [19,20,21] by which the hydrophobic distal end of the N-terminal -barrel domain dominantly interacts with HDLs via a hydrophobic interaction [22]
A plasma VLDL sample prepared from a healthy person with a normal TG level of 127 mg/dl was examined by both optimized negative stain (OpNS) [30, 31] electron microscopy (EM) and cryo-electron microscopy (cryo-EM) techniques
Summary
Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDLs can exchange their containing TGs with HDL CEs mediated by CE transfer protein (CETP) via a tunnel mechanism [19,20,21] by which the hydrophobic distal end of the N-terminal -barrel domain dominantly interacts with HDLs via a hydrophobic interaction [22]. It is unclear why this hydrophobic distal end has less interaction with the same types of surface lipids of VLDL. Cholesterol-enriched VLDL lipolytic remnants are associated with increased risk of CVD [23]
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