Characterization of coated-vesicle adaptors: their reassembly with clathrin and with recycling receptors.

Adaptors sort out those receptors that participate in assembly of coated pits from those that are excluded. Two distinct adaptor units have so far been identified: (1) adaptors restricted to plasma membrane coated pits (HA-II type, named according to their elution position during hydroxylapatite chromatography) and (2) adaptors restricted to Golgi region coated pits (HA-I type). Adaptors contain a heterodimer of two 100-kDa polypeptides, a beta-adaptin (possibly carrying an essentially common clathrin-binding domain) and a distinct alpha- or gamma-adaptin characteristic of the type of adaptor and its specific location. Each adaptor in constructed from four different polypeptides. Thus HA-II adaptors contain a beta-adaptin and an alpha-adaptin in combination with a 50-kDa protein and a 16-kDa polypeptide. The HA-I adaptors contain a beta-adaptin and a gamma-adaptin in combination with a 47-kDa protein and a 19-kDa polypeptide. Both types of adaptors and also a 180-kDa polypeptide will promote the assembly of clathrin to form coats, the size range of which appears to be relatively restricted compared to cages made from clathrin alone. The HA-II adaptors, characteristic of plasma membrane coated pits, bind to the cytoplasmic tail of the LDL receptor. They also assemble with the mannose 6-phosphate receptor in vitro in the absence of membrane. When clathrin is included, the adaptors promote the assembly of coats containing bound receptor.