Gene cloning and characterization of a GTP-binding Rab protein from mouse pituitary AtT-20 cells

The Rab proteins constitute a family of Ras-like, low-molecular-weight GTP-binding proteins that are involved in vesicular transport in mammalian cells. Several members of this family have been localized to specific cellular compartments, indicating that they may control vesicular transfer between discrete portions of the endomembrane systems. To examine the complexity of the Rab proteins produced in cells with both regulated and constitutive secretory pathways, we used a PCR-based strategy to clone members of this gene family from rodent pituitary. Here, we report the identification and studies of one of the clones, RAB18. The full-length cDNA clone from a mouse pituitary AtT-20 cDNA library encodes a 23.5-kDa protein with four consensus GTP-binding domains. The deduced amino acid sequence is 42% and 40% identical to Rab2 and Sec4p, respectively, and contains sequence motifs characteristic of the Rab family. Northern blot analysis shows that the level of expression is high in the brain, moderate in the pituitary, and low in the liver. Rab18 may be involved in membrane traffic events during biogenesis and/or recycling steps of secretory vesicle components.