Recycling of dissolved plasma membrane components as an explanation of the capping phenomenon

Abstract

IN a recent paper in Nature, Bretscher¹ proposed that the capping phenomenon^2,3 is caused by a continuous, directional flow of the lipid components of the plasma membrane across the cell surface from one end to the other, while the membrane proteins remain dispersed by random thermal motion and therefore do not normally participate. Bretscher's explanation of capping is that the cross linking of membrane proteins by attached antibodies and lectins causes them to be swept along by this lipid current and collected at the “uroid” portion of the cell surface, where he proposes that the membrane lipid is taken into the cytoplasm (by way of a “molecular filter”) for return to the opposite side of the cell as cytoplasmic vesicles. In this way Bretscher can account for the selective accumulation into caps of certain classes of surface antigen, without needing to postulate any communication between the surface and the interior. Cross-linked antigens would simply be swept into caps by the lipid current, while uncross-linked antigens would remain dispersed, just as is actually observed.