Studies of the cyclic adenosine monophosphate chemoreceptor ofParamecium

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Abstract
Summary A doublet of proteins (∼48,000M r) from theParamecium cell body membrane fits several criteria for the external cAMP chemoreceptor. These criteria include: (i) selective elution from a cAMP affinity column, matching a specificity that could be predicted from the behavioral response and whole-cell binding; (ii) binding to wheat germ agglutinin indicating the presence of carbohydrate moieties indicating surface exposure; and (iii) selective inhibition of the intact cells' chemoresponse to cAMP by antibodies against the doublet. Additional evidence for the existence of a receptor, in general, comes from selective elimination of the cAMP chemoresponse by photoaffinity labeling of whole cells with 8-N3-cAMP. The doublet proteins are not identical to the regulatory subunit of a cAMP-dependent protein kinase fromParamecium, theDictyostelium cAMP chemoreceptor, or the 42–45 kDa range proteins related to the large surface glycoprotein inParamecium. The doublet proteins are not readily separable and, as inDictyostelium, may represent two different covalent modification states of the same protein. Amino acid analysis indicates that the proteins are similar, but does not distinguish between the possibilities of proteolysis and covalent modification. Once cloned, this doublet may prove to be only the fifth external, eukaryotic chemoreceptor to be identified.