Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination.

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.