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Genetic evidence that cholera toxin substrates are regulatory components of adenylate cyclase
Abstract:
Cholera toxin, using [32P]NAD+ as substrate, specifically radiolabels at least two proteins in plasma membranes of wild type S49 mouse lymphoma cells. The toxin-specific substrates are detectable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis as bands corresponding to molecular weights of 45,000 and a doublet of 52,000 to 53,000. Membranes of two other cell types exhibit similar patterns of radiolabeled bands specifically produced by incubation with cholera toxin: the "uncoupled" variant S49 cell, which possesses adenylate cyclase activity unresponsive to hormones, and the HTC4 rat hepatoma cell, which lacks detectable catalytic adenylate cyclase activity but contains components of the cyclase system necessary for regulation by guanyl nucleotides and NaF. Little or no toxin-specific radiolabeling is observed in membranes of a fourth cell type, the adenylate cyclase activity-deficient S49 variant, which functionally lacks components of the cyclase system involved in cholera toxin action and regulation by guanyl nucleotides and NaF. The toxin-specific labeling pattern is not observed in membranes prepared from wild type S49 cells previously treated with cholera toxin in culture. One or both of the toxin substrates thus appears to be involved in regulation of adenylate cyclase by guanyl nucleotides and fluoride ion.
Insights
Cholera toxin specifically labels two proteins in cell membranes, identified as key players in regulating adenylate cyclase activity by guanyl nucleotides and fluoride ions.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Pharmacology
Background:
- Cholera toxin is a potent activator of adenylate cyclase.
- The precise molecular targets and regulatory roles of cholera toxin substrates in cellular signaling pathways remain incompletely understood.
Purpose of the Study:
- To identify and characterize cholera toxin-specific protein substrates in cell membranes.
- To investigate the role of these substrates in the regulation of adenylate cyclase activity.
Main Methods:
- Radiolabeling of plasma membrane proteins using [32P]NAD+ and cholera toxin.
- Analysis of radiolabeled proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).
- Comparison of labeling patterns across different cell types with varying adenylate cyclase activities.
Main Results:
- Cholera toxin specifically radiolabeled two major proteins (45,000 and 52,000-53,000 MW) in wild-type S49 mouse lymphoma cell membranes.
- Similar labeling patterns were observed in "uncoupled" S49 cells and HTC4 rat hepatoma cells, but not in adenylate cyclase-deficient S49 variants.
- Toxin-specific labeling was absent in membranes from cells pre-treated with cholera toxin.
- Pre-treatment with cholera toxin abolished toxin-specific labeling, suggesting substrate modification or depletion.
Conclusions:
- At least two cholera toxin substrates, with molecular weights of 45,000 and 52,000-53,000, are involved in the regulation of adenylate cyclase.
- These substrates are crucial components of the cellular machinery mediating cholera toxin action and are implicated in the regulatory effects of guanyl nucleotides and fluoride ions on adenylate cyclase activity.