Lipoprotein receptors: new roles for ancient proteins

Lipoprotein receptors used to be viewed simply as the means by which cells were supplied with lipids for energy production and membrane synthesis. Thi s perception has now changed dramatically. Megalin, a member of the low den sity lipoprotein receptor gene family, turns out to mediate the endocytic u ptake of retinoids and steroids, thus helping to regulate their biological function. Other members of this receptor family interact with cytosolic sig nalling proteins, giving this evolutionarily ancient family of receptors an entirely unexpected new role as transducers of extracellular signals.