Abstract
A novel rabphilin-3-like gene, granuphilin, has been identified in pancreatic beta cells by comparing genes expressed in pancreatic alpha and beta cell lines using mRNA differential display. The domain structure of the protein products of the granuphilin gene contains an amino-terminal zinc-finger motif and carboxyl-terminal C(2)-domains, similar to that of the rabphilin-3 gene. There are two isoforms: the larger isoform, granuphilin-a, has two C(2)-domains, whereas the smaller one, granuphilin-b, contains only the first C(2)-domain. Granuphilin is specifically expressed in pancreatic beta cells and the pituitary gland, but not in pancreatic alpha cells, the adrenal gland, or other major organs such as the brain. A portion of granuphilin associates with insulin-containing dense-core granules, but not with synaptic-like microvesicles in beta cells. Thus, its distribution pattern presents a striking contrast with that of rabphilin-3, which associates with small synaptic vesicles in neurons. The first C(2)-domain of granuphilin binds phospholipids in a Ca(2+)-independent manner, whereas the second one does not. These distinctive characteristics of granuphilin suggest that it is not a simple counterpart of rabphilin-3 in endocrine cells and that it has a unique role in the regulated exocytosis of dense-core granules in endocrine tissues.
Highlights
A combination of genetics in yeast and biochemistry in animal cell-free systems has revealed basic molecular mechanisms of constitutive exocytosis [1, 2]
Molecular Cloning of a Novel Rabphilin-3-like Gene— the pancreatic alpha and beta cells are developmentally very close, they have an opposite role in maintaining glucose homeostasis: the alpha cells secrete glucagon in response to a low glucose level, and the beta cells increase insulin secretion in response to a high glucose level
We have identified and characterized a novel rabphilin-3-like gene, granuphilin
Summary
A combination of genetics in yeast and biochemistry in animal cell-free systems has revealed basic molecular mechanisms of constitutive exocytosis [1, 2]. To analyze the molecular machinery of regulated exocytosis of LDCVs/granules, we studied pancreatic beta cells, which secrete insulin in response to blood glucose levels. Granuphilin, has a number of distinct characteristics compared with rabphilin-3: it is expressed in pancreatic beta cells and in the pituitary, but not in other major organs such as the brain.
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