Phosphorylation barcode-dependent signal bias of the dopamine D1 receptor

Abstract

Agonist-activated G protein-coupled receptors (GPCRs) must correctly select from hundreds of potential downstream signaling cascades and effectors. To accomplish this, GPCRs first bind to an intermediary signaling protein, such as G protein or arrestin. These intermediaries initiate signaling cascades that promote the activity of different effectors, including several protein kinases. The relative roles of G proteins versus arrestins in initiating and directing signaling is hotly debated, and it remains unclear how the correct final signaling pathway is chosen given the ready availability of protein partners. Here, we begin to deconvolute the process of signal bias from the dopamine D1 receptor (D1R) by exploring factors that promote the activation of ERK1/2 or Src, the kinases that lead to cell growth and proliferation. We found that ERK1/2 activation involves both arrestin and Gs, while Src activation depends solely on arrestin. Interestingly, we found that the phosphorylation pattern influences both arrestin and Gs coupling, suggesting an additional way the cells regulate G protein signaling. The phosphorylation sites in the D1R intracellular loop 3 are particularly important for directing the binding of G protein versus arrestin and for selecting between the activation of ERK1/2 and Src. Collectively, these studies correlate functional outcomes with a physical basis for signaling bias and provide fundamental information on how GPCR signaling is directed. Significance StatementThe functional importance of receptor phosphorylation in GPCR regulation has been demonstrated. Over the past decade, the phospho-barcode concept was developed to explain the multi-dimensional nature of the arrestin-dependent signaling network downstream of GPCRs. Here, we used the dopamine-1 receptor (D1R) to explore the effect of receptor phosphorylation on G protein-dependent and arrestin-dependent ERK and Src activation. Our studies suggest that D1R intracellular loop-3 phosphorylation affects both G proteins and arrestins. Differential D1R phosphorylation can direct signaling toward ERK or Src activation. This implies that phosphorylation induces different conformations of receptor and/or bound arrestin to initiate or select different cellular signaling pathways.