Uptake of cholesterol: an example of receptor-mediated endocytosis

Cholesterol is carried in the circulation in large spherical particles that originate in the liver, called low density lipoproteins (LDLs). Each LDL contains a non-polar core of cholesterol molecules esterified to long-chain fatty acids, surrounded by a lipid layer containing phospholipids, cholesterol and a single protein species, apoprotein B.

Cells that require cholesterol synthesize cell surface receptors (LDL receptor) that recognize apoprotein B specifically (Fig. 3.16.1). Within 10 min of binding, the LDL particle is internalized and delivered to the lysosomes, where the cholesterol is released from the cholesterol esters. LDL receptors are localized in clusters over small indentations or pits with a bristle-like appearance, termed coated pits.

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