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pubmed:abstractText |
It is well established that both GTP-binding proteins and phosphoproteins are involved in the control of exocytosis in the exocrine pancreas. Exocytotic membrane fusion is stimulated by guanosine 5'-[gamma-thio]triphosphate, and the phosphorylation states of several proteins, including at least one on the zymogen granule membrane, are known to change during exocytosis. We show here that a nucleoside diphosphate kinase is associated with the cytoplasmic face of pancreatic zymogen granules. This enzyme behaves as a phosphoprotein of apparent molecular mass 21 kDa on SDS/polyacrylamide gels, and is able to produce GTP by using ATP to phosphorylate endogenous GDP. GTP production by nucleoside diphosphate kinase is stimulated by the wasp venom peptide mastoparan, both through a direct action on the enzyme and through its ability to increase the availability of endogenous GDP. Two effects of the GTP produced by nucleoside diphosphate kinase are demonstrated: phosphorylation of a 37 kDa zymogen granule protein on histidine residues, and stimulation of the fusion of zymogen granules with pancreatic plasma membranes in vitro. These results suggest that granule-associated nucleoside diphosphate kinase is able to maintain local GTP concentrations, and raise the possibility that it might be involved in the control of exocytosis in the pancreatic acinar cell.
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