Inositol polyphosphate receptor and clathrin assembly protein AP-2 are related proteins that form potassium-selective ion channels in planar lipid bilayers

We have previously described an inositol polyphosphate receptor (IP_xRec), purified from detergent-solubilized bovine cerebellum microsomes, that displays potassium ion channel activity in planar lipid bilayers. We now find that the IP_xRec is closely related to clathrin assembly protein AP-2. The IP_xRec and AP-2 purified from bovine brain clathrin-coated vesicles share several structural and functional features: (i) similar subunit composition; each has four major polypeptides that have similar mobility (M_r values of 111,000, 100,000, 50,000, and 17,000) and relative intensity by SDS/PAGE analysis; (ii) similar size as studied by molecular sieve chromatography (M_r 400,000); (iii) identical N-terminal amino acid sequences for the M_r 50,000 subunits and M_r 111,000/100,000 doublets; (iv) immunoreactivity of the AP-2 M_r 111,000/100,000 doublet to polyclonal antibodies affinity purified against the doublet proteins of the IP_xRec; (v) display of the in vitro diagnostic feature of assembly proteins - i.e., they induce the assembly of clathrin cages; and (vi) ion channel activity selective for potassium ions with the same unitary conductance when incorporated into planar lipid bilayers. One difference was found. AP-2 channels were not blocked by inositol 1,3,4,5-tetraphosphate as reported for IP_x receptor channels. These studies suggest a possible connection between the IP_x signaling pathways and receptor-mediated endocytosis.