Identification of a clathrin binding subunit in the HA2 adaptor protein complex

The HA2 adaptor complex, comprising alpha-, beta-, 50-kDa, and 16-kDa subunits, was partially dissociated into its constituents with 3 M urea, and the beta-subunit was purified from the mixture by hydroxylapatite and affinity chromatography. The renatured beta-subunit behaves hydrodynamically as a single polypeptide of Mr approximately 128,000. In a sedimentation assay the purified beta-polypeptide co-sediments with pre-formed clathrin cages. The beta-polypeptide, however, will not induce assembly of clathrin triskelia. Our results support the conjecture that the beta-type subunits (beta and beta') of the HA2 and HA2 adaptor complexes serve to attach the HA-2 adaptor complex to clathrin (Ahle, S., Mann, A., Eichelsbacher, U., and Ungewickell, E. (1988) EMBO J. 7, 919-929), while the other subunits may determine the specificity of binding to docking proteins and receptors on cytoplasmic membrane surfaces.