Purification of the surface cAMP receptor in Dictyostelium

We have previously identified and demonstrated reversible ligand-induced modification of the major cell surface cAMP receptor in Dictyostelium discoideum. The receptor, or a subunit of it, has been purified to homogeneity by hydroxylapatite chromatography followed by two-dimensional preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification was monitored by following ³²P(i) incorporated by photoaffinity labeling with 8-azido-[³²P]cAMP or by in vivo labeling with ³²P(i). Two interconvertible forms of the receptor, designated R (M(r) 40,000) and D (M(r) 43,000), co-purified. Two-dimensional peptide maps of independently purified and ¹²⁵I-iodinated R and D forms of the receptor were nearly identical but did have several distinct peptides. The estimated 6000-fold purification required is consistent with the number of cell surface binding sites assuming there are not multiple binding sites/polypeptide. In the accompanying article we report the generation of a monospecific polyclonal antiserum which has helped to further elucidate the physical properties and developmental regulation of the cAMP receptor.