Some clathrin spots (red) remain in cells lacking AP-2.

The conventional view of clathrin-mediated endocytosis holds that the adaptor complex AP-2 is uniquely able to recruit clathrin to the plasma membrane to form coated pits. Using two different approaches, Motley et al. (page 909) and Conner and Schmid (page 773) now disprove this model. Instead of being an irreplaceable component of clathrin-coated pits and vesicles, AP-2 appears to be just one of several adaptors, and not all endocytic cargoes require it.Using siRNA, Motley et al. knocked expression of an AP-2 subunit down to undetectable levels and found that this inhibits the endocytosis of transferrin receptors, but surprisingly does not abolish coated pit formation or inhibit the internalization of EGF receptors or an LDL receptor chimera. Conner and Schmid found that overexpression of the adaptor associated kinase AAK1 in HeLa cells interferes with AP-2 function, apparently by sequestering AP2 complexes and preventing them from clustering on the plasma membrane. This also blocks transferrin endocytosis but does not stop coated pit formation or internalization of the EGF receptor. The results show that although some proteins require AP-2 for internalization, others do not, suggesting that alternative adaptors can drive coated pit formation and endocytosis. ▪