Spatial Regulation of Gαi Signaling in Clathrin-Coated Pits with GAIP

Fig. 2 Click to enlarge

G protein coupled receptors (GPCRs) transduce extracellular signals via heterotrimeric G proteins and trigger a variety of intracellular responses. Ligand binding to GPCRs causes activation of Gα and Gβγ subunits which regulate downstream effectors. Signaling is shut off by RGS (Regulators of G protein Signaling) proteins which serve as GAPs or GTPase-activating proteins that bind to Ga subunits through a conserved RGS domain (Fig. 1). The orchestration of these events and their localization at the cellular level is a topic of high current interest. Our laboratory has a long-standing interest in RGS proteins and in understanding their role in G protein signaling and trafficking. Some years ago we identified GAIP (RGS19)--a charter member of the RGS family--and demonstrated that it is located in clathrin-coated pits (CCPs) at the cell surface and in the Golgi (TGN). Gαi subunits are assumed to interact with GPCR at the cell membrane (PM). The localization of a GAP on CCPs raises the questions, Where does GAIP interact with Gαi and, what is its fate after receptor internalization?

Fig. 1 Click to enlarge

We recently investigated this question and our findings (Fig. 2) support a dynamic model for the spatial regulation of G protein signaling whereby activated Gai subunits together with activated DOR move from non-clathrin-coated to clathrin-coated microdomins where they bind GAIP, the inactivated (GTP-bound) Gαi move back to non-coated microdomains of the PM, and GPCR are down-regulated in lysosomes.

Thus our results suggest a novel mechanism for spatial regulation of Gαi signaling in clathrin-coated pits. Currently we are focused on further defining the mechanisms involved.