RAB5A, Member RAS Oncogene Family (RAB5A)

The S. cerevisiae YPT1 and SEC4 genes encode Ras-related GTP-binding proteins involved in the regulation of secretion. Mammalian cells express a large number of RAB proteins, GTP-binding proteins closely related to YPT1 and SEC4. Except for the closely related RAB3A and RAB3B, the deduced human RAB proteins share 32 to 50% homology. The predicted 214-amino acid RAB5 protein is 31% and 38% identical to SEC4 and YPT1, respectively. All 6 human RAB proteins tested bound GTP and exhibited GTPase activities in vitro. Northern blot analysis revealed that RAB5 was expressed as 2.7- and 2.8-kb mRNAs in a human fibroblast cell line. RAB5 is a rate-limiting component of the machinery regulating the kinetics of membrane traffic in the early endocytic pathway.