New roles for β-arrestins in cell signaling:: Not just seven-transmembrane receptors

beta-arrestins, originally discovered as molecules that bind to and desensitize the activated and phosphorylated form of the G protein-coupled beta(2)-adrenergic receptor (beta(2)-AR), have recently emerged as multifunctional adaptor/scaffold proteins that dynamically assemble a wide range of multiprotein complexes in response to stimulation of most seven-transmembrane receptors (7TMRs). These complexes mediate receptor signaling, trafficking, and degradation. Moreover, beta-arrestins are increasingly found to perform analogous functions for receptors from structurally diverse classes, including atypical 7TMRs such as frizzled and smoothened, the nicotinic cholinergic receptors, receptor tyrosine kinases, and cytokine receptors, thereby regulating a growing list of cellular processes such as chemotaxis, apoptosis, and metastasis.