The small GTP-binding protein rab4 is associated with early endosomes

Small GTP-binding proteins of the rab family have been implicated as playing important roles in controlling membrane traffic on the biosynthetic and endocytic pathways. We demonstrate that a distinct rab protein, rab4p, is associated with the population of early endosomes involved in transferrin-receptor recycling. An antibody to human rab4p was found to detect a doublet of approximately 24-kDa proteins on immunoblots from various cell types. Seventy-five percent of these proteins were tightly membrane bound and could be released only by detergent treatment. Upon isolation of early endosomes, late endosomes, and lysosomes, by free-flow electrophoresis and Percoll density-gradient centrifugation, most (70%) of the rab4p was found to cofractionate with early endosomes and endocytic vesicles containing 125I-labeled transferrin. The rab proteins previously localized to the endoplasmic reticulum and/or Golgi apparatus were not found in these fractions. We also localized rab4p to transferrin-receptor-containing early endosomes by immunofluorescence after expression of rab4 cDNA. The association of rab4p with early endosomes and other vesicles involved in the intracellular transport of transferrin receptor suggests that rab4p may play a role in regulating the pathway of receptor recycling.