Investigating a novel intracellular trafficking role for β-arrestin 1

Kathleen Webb, Julie Pitcher. MRC Laboratory for Molecular Cell Biology and Department of Neuroscience, Physiology and Pharmacology, University College London, Gower Street, London, WC1E 6BT, UK

β-Arrestins are best known for their role in mediating G protein-coupled receptor (GPCR) desensitisation, at least in part, through their ability to bind components of the endocytic machinery, including AP2, clathrin and Arf6, thereby promoting receptor internalisation. Here we describe a novel intracellular membrane trafficking role for β-arrestin 1 at the Golgi. Mouse embryonic fibroblasts (MEFs) derived from β-arrestin 1 knockout (KO), but not β-arrestin 2 KO or wild type (WT) mice, exhibit a fragmented Golgi morphology, a phenotype that is rescued upon β-arrestin 1 expression. Following the trafficking of soluble secretory DsRed (ssDsRed) in β-arrestin 1 KO as compared to WT MEFs shows a faster rate of anterograde secretory trafficking in the absence of β-arrestin 1. These observations suggest a role for β-arrestin 1 in attenuating trafficking and the associated vesiculation that occurs coincident with traffic flow through the Golgi. Co-immunoprecipitation studies reveal that β-arrestin 1 binds to the clathrin adaptor AP1 and Arf1. Notably, in marked contrast to the previously characterised role for Src in promoting dissociation of β-arrestin 1/AP2 complexes at the plasma membrane here we show here that Src potentiates β-arrestin 1/AP1 complex formation. High Src activity is associated with rapid Golgi transport rates and with a fragmented Golgi phenotype. It is thus tempting to speculate that β-arrestin 1 may serve to attenuate anterograde trafficking in a Src-dependent fashion. Notably, the fragmented Golgi morphology observed in the HT29 colorectal cancer cell line, which has high endogenous levels of active Src, is compacted by overexpression of β-arrestin1, but not β-arrestin 2. We are currently assessing a variety of biochemical secretory trafficking assays that will be used to probe the role of β-arrestin 1 in regulating secretory traffic in more detail. It is anticipated that the ability of β-arrestin 1 mutant constructs to regulate traffic flux may provide information concerning the molecular players involved in this process. How Golgi morphology is maintained in the face of continual and varied traffic flux is a fundamental problem in cell biology, our results point to a role of β-arrestin 1 in this process.