Rab4 is phosphorylated by the insulin‐activated extracellular‐signal‐regulated kinase ERK1

Abstract

Rab4, a low‐molecular‐mass GTP‐binding protein, is associated with vesicles containing Glut 4 in adipocytes. Following insulin stimulation, the translocation of Glut 4 to the plasma membrane is associated with the movement of Rab4 to the cytosol. The same modifications are induced by the phosphatase inhibitor, okadaic acid. To establish a possible role for phosphorylation in Rab4 cycling, we searched for insulin‐stimulated cytosolic kinase(s) which could phosphorylate Rab4. In 3T3‐L1 adipocytes, insulin induced a rapid and transient activation of cytosolic kinase(s), which phosphorylated Rab4 in vitro. At least part of the Rab4 phosphorylation can be accounted for by ERK (extracellular‐signal‐regulated kinases) since immunopurified ERK1 from insulin‐stimulated cells phosphorylated Rab4 with a comparable time‐course. Both with cytosolic extracts and immunopurified ERK1, only serine residues were phosphorylated on Rab4. The phosphorylation site was localized in the C‐terminus of the molecule, and occurred very probably on Ser196. These results indicate that Rab4 is an in vitro substrate for ERK, and suggest that the insulin‐induced movement of Rab4 from the Glut‐4‐containing vesicles to the cytosol could result from phosphorylation of Rab4 by ERK. Copyright © 1994, Wiley Blackwell. All rights reserved