Molecule of the Month: Apolipoprotein B-100 and LDL Receptor

Lipoproteins are protein-lipid particles that travel in the bloodstream and act as delivery vehicles for diverse lipids including triglycerides, cholesterol, and phospholipids. These fats are needed by tissues throughout the body to maintain essential functions, such as building and maintaining lipid bilayers. However, the levels of lipoprotein particles in circulation must be carefully regulated; a diversity of health issues can arise if lipoproteins are allowed to accumulate.

The journey of most lipoproteins begins in the liver, where specialized cells produce and release particles known as very low-density lipoproteins (VLDLs) that are packed with triglycerides, cholesterol (and cholesterol derivatives) and phospholipids, and proteins known as apolipoproteins. As VLDL circulates in the bloodstream, cells in various tissues extract triglycerides for energy or storage, gradually shrinking and altering the particle's composition. This process ultimately transforms VLDL into a low-density lipoprotein (LDL), which is enriched in cholesterol relative to VLDLs. The protein composition of the lipoprotein also changes, with different apolipoproteins being exchanged or removed from the particle during circulation. Apolipoprotein B-100 (or ApoB100), however, which is a large amphipathic protein that forms a belt-like structure (shown in orange in the image to the right, PDB 9BDT), remains tightly bound to the particle throughout its journey from VLDL to LDL.

Circulating LDL eventually returns to the liver, where it is absorbed and broken down by liver cells. These cells express specialized LDL receptors (shown in purple and pink) that recognize and bind to ApoB100. Once bound, the LDL particle is engulfed by the cell through a process called endocytosis.

The acidic environment of the endosome triggers release of the LDL particle. This process is thought to occur through the protonation of the LDL receptor, which decreases its affinity for the LDL particle and increases its affinity for itself. The LDL receptor folds back on itself, as shown in the figure on the right (PDB 1N7D), and releases the LDL particle into the vesicle lumen. The LDL particle is then trafficked to the lysosome or other cellular compartments, while the LDL receptor is recycled back to the plasma membrane.

When VLDL is first released by liver cells, it measures around 30–60 nm in diameter and is packed with triglycerides. By the time it is transformed into LDL and endocytosed, its size has shrunk to about 20 nm. Remarkably, throughout this journey, ApoB100 remains tightly wrapped around the lipoprotein, flexibly adapting to its changing size. Recent insights from an integrative modeling approach—combining data from cryo-EM, structural predictions, and molecular dynamics simulations—reveal how this flexibility is achieved. The central beta belt of ApoB100 can adjust by tightening or loosening, with its ends sliding past one another to accommodate the particle as it shrinks. Flexible interstrand inserts within ApoB100 can bend and contact other ApoB100 segments when the particle is at its smallest, but can easily accommodate larger particles by straightening bent regions and releasing contacts. This structural adaptability allows ApoB100 to remain wrapped around the lipoprotein as it changes in size and composition.