Don’t Let GPCR Tagging Bias Your Results

A Focus on β-arrestin

One area of rapidly expanding knowledge is the role β-arrestins play in mediating GPCR signaling. As the name suggests, GPCRs were originally thought to initiate signal transduction via associated G proteins, while β-arrestin was believed to play a more inhibitory role involved in the desensitization of GPCRs.

Subsequent research has shown that β-arrestins, both in conjunction with and independently of G proteins, mediate a number of ligand activated GPCR activities including receptor internalization, trafficking, and receptor desensitization as well as initiation of signaling via distinct kinase cascades including MAPK and PI3K. One commonality is that all β-arrestin-mediated activities begin with translocation of β-arrestin to the activated GPCR. 

Upon activation, GPCRs interact with three main families of proteins – G proteins, GPCR kinases (GRKs), and arrestins. These interactions result in bimodal GPCR activation: G protein-mediated activation and/or GRK and β-arrestin-dependent activation. Ligands may stimulate both G protein- and β-arrestin- mediated modes of signaling or preferentially use one mode over another, an effect known as biased signaling.

Studies have revealed that GPCR conformational changes and GRK phosphorylation patterns upon ligand binding are responsible not only for initiating cellular signaling, but also for determining which signaling mode(s) is stimulated. One means of regulating signaling pathway usage is via the recruitment and differential phosphorylation of the carboxy-terminal tail of ligand-activated GPCRs by different GRK isoforms. The resulting pattern of phosphorylation, often referred to as the phosphorylation ‘barcode’, mediates subsequent cellular outcomes, including β-arrestin involvement.