Members of the Ras-related superfamily of GTP binding proteins, which includes Ras, Rho, Rab and ARF subfamilies, exhibit 30-50% similarity with Ras p21. Rab proteins play an important role for either in endocytosis or in biosynthetic protein transport. The possibility that Rab proteins might also direct the exocytosis from secretory vesicles to the plasma membrane is supported by the observation that in yeast, the SEC4 protein, which is 40% similar to Rab proteins, is associated with secretory vesicles.
Rab proteins located on the cytoplasmic face of organelles and vesicles, rab proteins are involved in intracellular membrane fusion reactions. Rab25 was cloned from a gastric parietal cell cDNA library and is expressed in epithelial tissues such as the gastrointestinal mucosae, kidney, and lung, which encoded a protein of 28 kDa