Glucose- and GTP-dependent stimulation of the carboxyl methylation of CDC42 in rodent and human pancreatic islets and pure β cells. Evidence for an essential role of GTP-binding proteins in nutrient-induced insulin secretion

Several GTP-binding proteins (G-proteins) undergo posttranslational modifications (isoprenylation and carboxyl methylation) in pancreatic β cells. Herein, two of these were identified as CDC42 and rap 1, using Western blotting and immunoprecipitation. Confocal microscopic data indicated that CDC42 is localized only in islet endocrine cells but not in acinar cells of the pancreas. CDC42 undergoes a guanine nucleotide-specific membrane association and carboxyl methylation in normal rat islets, human islets, and pure β (HIT or INS-1) cells. GTPγS-dependent carboxyl methylation of a 23- kD protein was also demonstrable in secretory granule fractions from normal islets or β cells. AFC (a specific inhibitor of prenyl-cysteine carboxyl methyl transferases) blocked the carboxyl methylation of CDC42 in five types of insulin-secreting cells, without blocking GTPγS-induced translocation, implying that methylation is a consequence (not a cause) of transfer to membrane sites. High glucose (but not a depolarizing concentration of K⁺) induced the carboxyl methylation of CDC42 in intact cells, as assessed after specific immunoprecipitation. This effect was abrogated by GTP depletion using mycophenolic acid and was restored upon GTP repletion by coprovision of guanosine. In contrast, although rap 1 was also carboxyl methylated, it was not translocated to the particulate fraction by GTPγS; furthermore, its methylation was also stimulated by 40 mM K⁺ (suggesting a role which is not specific to nutrient stimulation). AFC also impeded nutrient-induced (but not K⁺-induced) insulin secretion from islets and β cells under static or perifusion conditions, whereas an inactive structural analogue of AFC failed to inhibit insulin release. These effects were reproduced not only by S- adenosylhomocysteine (another methylation inhibitor), but also by GTP depletion. Thus, the glucose- and GTP-dependent carboxyl methylation of G- proteins such as CDC42 is an obligate step in the stimulus-secretion coupling of nutrient-induced insulin secretion, but not in the exocytotic event itself. Furthermore, AFC blocked glucose-activated phosphoinositide turnover, which may provide a partial biochemical explanation for its effect on secretion, and implies that certain G-proteins must be carboxyl methylated for their interaction with signaling effector molecules, a step which can be regulated by intracellular availability of GTP.