Novel function of POSH, a JNK scaffold, as an E3 ubiquitin ligase for the Hrs stability on early endosomes

POSH (plenty of SH3s) acts as a scaffold that links activated Rac1 and downstream c-Jun N-terminal kinase (JNK) signaling modules. However, it is unknown whether it's functional domain-mediated roles including the interesting RING-finger domain or its cellular function. Here, we provide evidence that subcellular localization of POSH is regulated by a particular domain of the protein and POSH was colocalized with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) on early endosomes via interaction of Hrs with POSH's two rear SH3 domains. Moreover, the RING domain of POSH specifically regulates the stability of Hrs, but not of JNK1, via a ubiquitin-proteasomal degradation pathway. Finally, we demonstrate that JNK1 does not interact with Hrs under the conditions of POSH interacted with Hrs, but instead reduces the POSH-catalyzed ubiquitination of Hrs and their reciprocal interaction. Together, these data suggest that POSH has a distinct role as a specific E3 ubiquitin ligase for Hrs on early endosomes, and there exists a relationship between its separate activities as a scaffold and as an E3.