Abstract

S49 mouse lymphoma cells resistant to killing by cholera toxin (but sensitive to N⁶,O²'-dibutyryl adenosine-3',5'-monophosphate) can be selected in a single step. The transition from cholera toxin sensitivity to resistance is stochastic and occurs at a rate of 1.1 x 10^-5 per cell per generation. Chemical mutagens increase the frequency of cholera toxin resistance. Screening of independently selected cholera toxin-resistant clones led to discovery of three novel variant phenotypes. Lesions in two of these phenotypes affect the guanine nucleotide regulatory component, called N, of adenylate cyclase (EC 4.6.1.1). We assessed the N protein in the variants by measuring the ability of membrane extracts to complement N-deficient S49 cyc^- membranes in vitro, and by radiolabeling peptide subunits of N in the presence of cholera toxin and [³²P]NAD⁺. Membranes of one variant phenotype, termed N^par, contain about 20% of the N activity seen in wild-type (parental) S49 membranes and show reduced radiolabeling catalyzed by cholera toxin. N^par membrane extracts partially inhibit wild-type extracts in complementing adenylate cyclase of cyc^- membranes. A second phenotype exhibits cholera toxin-specific radiolabeling of N subunits comparable to that seen in wild type, but its N protein exhibits very little activity in complementing the defect of cyc^-. Resistance to cholera toxin in the third phenotype is associated with normal N and adenylate cyclase activities, but extracts of these cells degrade adenosine 3',5'-monophosphate at a rate 4 times faster than wild-type.