Peptide mapping of fat cell membrane substrates for cholera toxin-catalyzed ADP-ribosylation

Incubating rat fat cell membranes with [32P]NAD+ and cholera toxin results in ADP-ribosylation of three distinct components with approximate molecular weights of 42,000, 46,000 and 48,000. Partial proteolytic peptide maps of the Mr = 46,000 and 48,000 toxin-specific substrates generated by elastase, alpha-chymotrypsin, or Staphylococcus aureus V-8 protease were nearly identical, while those of the Mr = 42,000 target lacked several peptides common to both of the larger molecular weight targets. In addition, peptide maps generated from the Mr = 42,000 target displayed a number of peptides which were absent from the maps generated from either the Mr = 46,000 or 48,000 targets. These data suggest that the Mr = 46,000 and 48,000 substrates are closely related proteins, however the relationship between the Mr = 42,000 toxin-specific substrate and the larger peptides remains to be established. The relative patterns of fat cell membrane labelling by cholera toxin in the presence of [32P]NAD+ were identical in hypothyroid as compared to euthyroid rat fat cells.