Recruitment of plasma membrane-localized cargo into clathrin-coated endocytic vesicles is mediated by interaction with a variety of clathrin-interacting proteins collectively called CLASPs (clathrin-associated sorting proteins). CLASP proteins, which may be monomeric or tetrameric, are recruited to the plasma membrane through interaction with phosphoinsitides and recognize linear or conformational sequences or post-translational modifications in the cytoplasmic tails of the cargo protein. Through bivalent interactions with clathrin and/or other CLASP proteins, they bridge the recruitment of the cargo to the emerging clathrin coated pit (reviewed in Traub and Bonifacino, 2013). The tetrameric AP-2 complex, first identified in early studies of clathrin-mediated endocytosis, was at one time thought to be the primary CLASP protein involved in cargo recognition at the plasma membrane, and indeed plays a key role in the endocytosis of cargo carrying dileucine- or tyrosine-based motifs.
A number of studies have been performed to test whether AP-2 is essential for all forms of clathrin-mediated endocytosis (Keyel et al, 2006; Motely et al, 2003; Huang et al, 2004; Boucrot et al, 2010; Henne et al, 2010; Johannessen et al, 2006; Gu et al, 2013; reviewed in Traub, 2009; McMahon and Boucrot, 2011). Although depletion of AP-2 differentially affects the endocytosis of different cargo, extensive depletion of AP-2 through RNAi reduces clathrin-coated pit formation by 80-90%, and the CCPs that do form still contain AP-2, highlighting the critcical role of this complex in CME (Johannessen et al, 2006; Boucrot et al, 2010; Henne et al, 2010).
In addition to AP-2, a wide range of other CLASPs including proteins of the beta-arrestin, stonin and epsin families, engage sorting motifs in other cargo and interact either with clathrin, AP-2 or each other to facilitate assembly of a clathin-coated pit (reviewed in Traub and Bonifacino, 2013).