Regulation of HRas signaling by Rabex‐5‐mediated ubiquitination

Abstract

Mammalian cells express three isoforms of Ras proteins, HRas, NRas and KRas4B (referred to as KRas hereafter). Each isoform plays an important role in controlling a wide spectrum of fundamental cellular processes. We have recently shown that the location‐specific signaling activity of HRas and NRas but not KRas is regulated by ubiquitination. The objective of this study is to identify the molecular mechanism by which Ras ubiquitination is regulated. Using the in vivo and in vitro ubiquitination assays along with RNA interference technology, we have found that the E3 ligase Rabex‐5 (also known as RabGEF1), but not its ligase‐deficient counterpart (Y25A/Y26A), is necessary and sufficient to catalyze Ras ubiquitination. Rabex‐5‐mediated HRas ubiquitination promotes its endosomal localization and leads to the suppression of ERK activation. Furthermore, the Ras effector RIN1 enhances HRas ubiquitination through its Rab5 GEF activity in a Rabex‐5 dependent manner. Thus RIN1 and Rabex‐5‐dependent Ras ubiquitination represents a new modality for the regulation of Ras compartmentalization and signaling.This work was supported by research grants from National Institutes of Health to D.B.‐S. (CA55360 and GM078266).