Ligand-induced modification of a surface CAMP receptor of Dictyostelium diswideum does not require its occupancy

duced phosphorylation of the surface cAMP receptor is associated with a discrete transition in its electro-phoretic mobility. The native and modified forms of the receptor are designated R and D (Mr = 40,000 and 43,000). The relationship of the number of receptors which are modified as a function of the receptors which bind cAMP was investigated. Modification was as-sessed by determining the amounts of R and D forms in Western blots which detect all receptors whether or not they are exposed on the surface. Cyclic AMP or the analog, adenosine 3‘,5’-monophosphorothioate ((R,)-CAMPS), induced a loss of CAMP-binding activity (down-regulation), which was not accompanied by a loss of the receptor protein. About 60 % of the receptors do not bind cAMP in th