Regulation of Protein Trafficking by α-arrestins
Nearly half of all prescription drugs alter G-protein coupled receptor (GPCR) signaling, including treatments for asthma, hypertension, neurodegenerative disorders and depression. β-arrestins are critical regulators of GPCRs: they act as trafficking adaptors to control GPCR endocytosis and impede G-protein signaling. β-arrestins are themselves therapeutic targets, highlighting the clinical importance of understanding arrestin function. However, β-arrestins are only a small branch of the larger arrestin family that includes the widely-conserved but functionally uncharacterized α-arrestins, the primary focus of our research. Our work has shown that α-arrestins, like β-arrestins, regulate GPCR signaling, but also operate in unexpected trafficking pathways, including endosomal recycling and clathrin-independent endocytosis. Using Saccharomyces cerevisiae as a model, our research has shown that α-arrestins interact with signaling regulators, including kinases and phosphatases, to selectively direct trafficking of specific transmembrane cargo proteins, presumably by promoting their ubiquitination and packaging into vesicles (Figure 1), and have begun to define the molecular mechanisms underlying α-arrestin-mediated trafficking. All of the α-arrestin-interacting partners identified in yeast are conserved. Our research will apply insights gained in yeast to initiate studies on the relatively unstudied mammalian α-arrestins.
The study of α-arrestins is in its infancy. There are still many unanswered questions about arrestin biology: What are the initial signaling cues that regulate α-arrestin trafficking? How are specific cargo proteins recognized? How does the arrestin-cargo interaction direct a protein cargo to its final destination? Our research employs molecular, biochemical, genetic and advanced microscopy methods to address these fundamental questions about arrestin function in yeast to expand our understanding of GPCR signaling and protein trafficking.
Current Lab Research
1. Delineating the signaling pathways that regulate α-arrestins
Arrestins respond to signaling cues to selectively bind and regulate trafficking of their membrane cargo (Figure 2). Phospho-regulation of α-arrestins is complex, with over 25 phospho-sites on a single α-arrestin. We have identified protein phosphatases and kinases that act as regulatory switches to control α-arrestin-mediated trafficking. Our work suggests that phosphorylation inhibits the endocytic function of α-arrestins while promoting intracellular sorting. In addition, ubiquitination of α-arrestins also has a role in regulating α-arrestin-mediated trafficking, and we are currently mapping α-arrestin ubiquitination sites using mass spectrometry and assessing their impact on trafficking functions.
2. Defining trafficking functions for α-arrestins
While α-arrestins have a well-established role in regulating clathrin-mediated endocytosis (Figure 1), we have expanded their repertoire of function identifying a new role for these adaptors in clathrin-independent endocytosis (Figure 3) as well as intracellular sorting of protein cargo. How α-arrestins contribute to these and other new protein trafficking pathways is an area of active investigation in the lab.
3. Identifying new α-arrestin cargos
Using a computational biology approach, we are generating a comprehensive map of α-arrestin-cargo pairs. To date, we have identified many new membrane proteins whose trafficking is regulated by α-arrestins and we will leverage these findings to define α-arrestin-cargo interaction interface.