Very low-density lipoproteins (VLDL) are metabolic precursors of low-density lipoproteins (LDL) and a risk factor for atherosclerosis. Human VLDL are heterogeneous complexes containing triacylglyceride-rich apolar lipid core and polar surface comprised of phospholipids, a nonexchangeable apolipoprotein B, and exchangeable apolipoproteins E and Cs. We report the first stability study of VLDL. Circular dichroism and turbidity data reveal an irreversible heat-induced VLDL transition that involves formation of larger particles and repacking of apolar lipids but no global protein unfolding. Heating rate effect on the melting temperature indicates a kinetically controlled reaction with high activation energy, E a . Arrhenius analysis of the turbidity data reveals two kinetic phases with E a =53±7 kcal/mol that correspond to distinct morphological transitions observed by electron microscopy. One transition involves VLDL fusion, partial rupture and dissociation of small spherical particles (d=7-15 nm), and another involves complete lipoprotein disintegration and lipid coalescence into droplets accompanied by dissociation of apolipoprotein B. The small particles, which are unique to VLDL denaturation, are comparable in size and density to high-density lipoproteins (HDL); they have apolar lipid core and polar surface comprised of exchangeable apolipoproteins (E and possibly Cs) and phospholipids. We conclude that, similar to HDL and LDL, VLDL are stabilized by kinetic barriers that prevent particle fusion and rupture and decelerate spontaneous inter-conversion among lipoprotein classes and subclasses. In addition to fusion, VLDL disruption involves transient formation of HDL-like particles that may mimic protein exchange among VLDL and HDL pools in plasma.Plasma lipoproteins, including high-, low-, intermediate-, and very-low density lipoproteins (HDL, LDL, IDL and VLDL), are macromolecular complexes of lipids and proteins (termed apolipoproteins) that mediate lipid transport and metabolism and are central in the development of coronary artery disease. HDL are anti-atherogenic, LDL are pro-atherogenic, and VLDL are not only direct metabolic precursors of LDL but also an independent risk factor for atherosclerosis (1)(2)(3)(4)(5)(6)(7)(8). VLDL are the major carriers of triacylglycerides (TG) in plasma. Human VLDL form heterogeneous population of spherical particles that contain apolar core comprised mainly of TG and cholesterol esters (CE) and polar surface comprised of cholesterol-containing phospholipid monolayer and proteins. The proteins include one copy of non-exchangeable apolipoprotein B (apoB, 550 kD) and multiple copies of exchangeable apolipoproteins, mainly apoE (34 kD) and apoCs (6-9 kD), that comprise over 50% of the total VLDL protein content. Metabolic remodeling by lipolytic enzymes converts VLDL into LDL that contain apoB as their sole protein, while the dissociated apoE and apoCs enter the HDL pool (2,4). Structural Our recent thermal and chemical denaturation analyses have shown that HDL and LDL...

Lipid droplets (LDs) in all eukaryotic cells are coated with at least one of the perilipin (Plin) family of proteins. They all regulate key intracellular lipases but do so to significantly different extents. Where more than one Plin is expressed in a cell, they associate with LDs in a hierarchical manner. In vivo, this means that lipid flux control in a particular cell or tissue type is heavily influenced by the specific Plins present on its LDs. Despite their early discovery, exactly how Plins target LDs and why they displace each other in a “hierarchical” manner remains unclear. They all share an amino-terminal 11-mer repeat (11mr) amphipathic region suggested to be involved in LD targeting. Here, we show that, in vivo, this domain functions as a primary highly reversible LD targeting motif in Plin1–3, and, in vitro, we document reversible and competitive binding between a wild-type purified Plin1 11mr peptide and a mutant with reduced binding affinity to both “naked” and phospholipid-coated oil–water interfaces. We also present data suggesting that a second carboxy-terminal 4-helix bundle domain stabilizes LD binding in Plin1 more effectively than in Plin2, whereas it weakens binding in Plin3. These findings suggest that dual amphipathic helical regions mediate LD targeting and underpin the hierarchical binding of Plin1–3 to LDs.

An actin regulatory protein unexpectedly also controls microtubule polymerization during the formation and maintenance of cellular outgrowths in neurons.