ATP is Required for Receptor-Mediated Endocytosis Both in Vivo and in Vitro

Abstract

The process of receptor-mediated endocytosis via clathrin-coated pits has been well-defined at the ultrastructural level. Receptors are concentrated in specialized “coated pit” regions of the cell surface which invaginate and pinch-off to form coated vesicles carrying receptors and bound ligands into the cell. At the molecular level, most of the constituents of the protein coat are known (although new coat constituents are still being identified, Ahle and Ungewickell, 1990) and a growing number of these have been cloned allowing primary sequence determination (for reviews see Pearse and Crowther, 1987; Brodsky, 1988; Morris et al., 1989). The major coat proteins are clathrin triskelions which consist of three 180kD clathrin heavy chains each with a tightly associated ∼30kD light chain. Assembly protein (AP) complexes or ‘adaptins’ appear to mediate the binding of the clathrin coat to the plasma membrane perhaps through direct interaction with the cytoplasmic domains of receptors (Keen, 1987; Virshup and Bennett, 1988; Glickman et al. 1989). These complexes consist of two ∼110kD, two ∼50 kD and two ∼20 kD subunits and are present at a 3:2 molar ratio to triskelions in the coat (reviewed by Morris et al., 1989). Two distinct protein kinases have also been identified as constituents of the coat structure, the 50kD kinase subunit of the assembly protein complex API and a casein kinase II-activity. An additional light chain B kinase activity has also been identified. No function has yet been ascribed to either of these kinase activities.