AP2 as a Recruiter of Clathrin

Clathrin plays a key role in endocytosis by functioning as a molecular scaffold that drives formation of transport vesicles, in part by stabilizing curved membrane domains (Hinrichsen et al. 2006). In solution clathrin forms a stable heterohexameric complex consisting of three heavy and three light chains organized into a three-legged "triskelion." The triskelia are thought to multimerize into polyhedral cages that provide the mechanical force to form vesicles from a membrane. Although purified clathrin can self-assemble into cages, its recruitment to membranes and assembly in vivo is regulated by a number of clathrin-binding proteins including AP-2 well as a variety of AP-2-associated accessory factors, such as amphiphysin, epsins, AP180/CALM, etc.

Interactions with clathrin are generally accomplished through simple linear arrangements of peptide motifs harboring hydrophobic and usually acidic residues (Dell'Angelica 2001) within natively unfolded domains. Analysis of clathrin-binding motifs in a variety of endocytic proteins has uncovered two classes of motifs: type-I and type-II clathrin boxes. Type-I motifs conform to the consensus sequence L0x0 (with 0 representing a bulky hydrophobic residue); type-II motifs are represented by the motif L0DLL. Both types of motifs interact with the N-terminal P-propeller domain of the clathrin heavy chain and promote clathrin cage assembly in vitro and presumably also in vivo (Maldonado-Baez and Wendland 2006).

Recent studies have revealed that AP-2 in addition to the known type-I clathrin box within the hinge domain of its P2 subunit (i.e., LLNLD) harbors a second clathrin binding site within the P2 appendage domain (Edeling et al. 2006a). Mutations in this site reduced AP-2 binding to clathrin, suggesting that both sites need to cooperate for stable association with clathrin. The relative importance of AP-2 as opposed to other endocytic proteins in getting clathrin to membranes is somewhat uncertain. RNAi-mediated knockdown of AP-2 leads to a strong reduction of clathrin-coated pits at the plasma membrane, suggesting that AP-2 may be a key factor in clathrin recruitment (Hinrichsen et al. 2003; Motley et al. 2003).

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