In 1975, coated vesicles, which transport many membrane proteins, were purified and shown to include an ~180-kDa protein named clathrin (Pearse 1975). Later, other molecules involved in clathrin binding to the vesicles were also identified (Keen et al. 1979). In 1987, Keen et al. showed that these molecules formed heterotetrameric complexes, which were termed as adaptor protein complex (AP)-1 and AP-2 (Keen 1987). AP-1 and AP-2 are composed of two large subunits (γ and β1 for AP-1 and α and β2 for AP-2), one medium subunit (μ1 for AP-1 and μ2 for AP-2), and one small subunit (σ1 for AP-1 and σ2 for AP-2). Later, another AP complex AP-3 was identified on the basis of its structural homology to AP-1 and AP-2 (Pevsner et al. 1994; Newman et al. 1995). The AP-3complex, which consists of one molecule each of δ, β3, μ3, and σ3 subunits, is involved in protein trafficking and organelle biogenesis...
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