SR-BI is required for microvillar channel formation and the localization of HDL particles to the surface of adrenocortical cells in vivo.

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of HDL cholesteryl ester into liver and steroidogenic tissues. In steroidogenic cells, juxtaposed microvilli, or microvilli snuggled against the plasma membrane create microvillar channels that fill with HDL. Microvillar membranes contain SR-BI and are believed to be the site of HDL cholesteryl ester uptake. A recent study showed that SR-BI expression in insect cells elicits membrane structures that contain SR-BI, bind HDL, and closely resemble the ultrastructure of microvillar channels. In the present study we compared the ultrastructure of adrenal gland microvillar membranes in Srb1+/+ and Srb1-/- mice to test whether SR-BI is required for the formation of microvillar channels. The results show that SR-BI is absolutely required for microvillar channel formation and that the microvillar membranes of Srb1-/- mice are 17% thinner than in Srb1+/+ mice. We conclude that SR-BI has a major influence on plasma membrane ultrastructure and organization in vivo.