Identification and the role of low molecular weight GTP-binding protein in the rat parotid acinar cells

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Low moledular weight GTP-binding proteins, Rab4, Rab3D and Rab26, were mainly detected by RT-PCR techniques using mRNA isolated from the rat parotid gland. Rab26 and VAMP2 (vesicle associated membrance protein 2) were detected in secretory granule membrane, but neither in the supernatant nor the plasma membane fractions. On the other hand, Rab4 was mainly detected in the plasma membrane fraction, but not in the granule membrane. After acinar cells were treated with isoproternol, VAMP2 was transferred to the intracellular particles from the granule membrance. However, Rab 26 was disappeared from the granule membrance, and not detected in the acinar cells. The results obtained using western blotting were also confirmed by immuno-histochemistry. These finding suggest that Rab26 participates in the regulated secretion of granules and functionally belongs to the Rab26 group.Treatment of acinar cells with carbachol partially transferred Rab4 to the plasma membrane from the intracellular particle. We have just started to reveal the role of Rab4 on the transportation of glucose transporter and/or aquaporin 5, so far we do not obtained enough data for reporting.In the parotid secretion, beta-agonist stimulates salivary secretion without change of the intracellular calcium ion. And rabphilin, which binds Rab3A, was not detected in the parotid gland, suggesting that the secretory mechanisms in the parotid are different from the calcium dependent pathway. We proposed mechanisms that PRA (prenylated Rab associated protein) regulates the secretion through phosphorylation.