Mdm2-mediated ubiquitination of β-arrestin2 in the nucleus occurs in a Gβγ- and clathrin-dependent manner

The fate and activity of β-arrestin2, a key player in the regulation of desensitization and endocytosis of G protein-coupled receptors (GPCRs), are regulated by mouse double minute 2 homolog (Mdm2)-mediated ubiquitination. However, details of the molecular mechanisms of β-arrestin2 ubiquitination remain unclear. Studies on β-arrestin2 and Mdm2 mutants with modified nucleocytoplasmic shuttling properties have revealed that β-arrestin2 ubiquitination occurs in the nucleus in a Gβγ- and clathrin-dependent manner. The nuclear entry of both β-arrestin2 and Mdm2 commonly relies on the presence of importin complex but can occur independently of each other. Gβγ and clathrin regulated the nuclear entry of β-arrestin2 by mediating the interaction between β-arrestin2 and importin β1. In contrast, Akt-mediated phosphorylation of two serine residues of Mdm2 partly regulated the nuclear entry of Mdm2. Ubiquitinated β-arrestin2 along with Mdm2 translocated to the cytoplasm where they play various functional roles including receptor endocytosis and ubiquitination of other cytoplasmic proteins. The nuclear export of Mdm2 required nuclear entry and interaction of β-arrestin2 with Mdm2. Ubiquitination was required for the translocation of β-arrestin2 toward activated receptors on the plasma membrane and for its endocytic activity. The current study revealed the cellular components and processes involved in the ubiquitination of β-arrestin2, and these findings could be quintessential for providing directions and detailed strategies for the manipulation of GPCR functions and development of GPCR-related therapeutics.