The G protein-coupled receptor kinase (GRK2) belongs to a family of proteins that phosphorylates agonist-activated G protein-coupled receptors, leading to G protein-receptor uncoupling and termination of G protein signaling. This kinase has been implicated in the development of essential hypertension. We have made a novel finding that GRK2 regulates activity of the epithelial Na+ channel (ENaC), a heterotrimeric Na+ transport protein expressed in the distal nephron, the distal colon and ducts of salivary and sweat glands and which plays an important role in Na+ homeostasis and the regulation of blood pressure. Inhibition of GRK2 activity in isolated mouse mandibular duct cells attenuated the activity of ENaC (Dinudom et al., 2004), suggesting that basal activity of this kinase is important to maintain an appropriate level of Na+ absorption in these epithelial cells. In the same tissue, a recombinant GRK2 and a phosphatase inhibitor, okadeic acid, interrupted feedback inhibition on ENaC by cytosolic Na+. This was due to the inhibitory effect of GRK2 on Nedd4-2, an ubiquitin protein ligase that facilitates endocytosis of ENaC, a key component of the Na+ feedback mechanism. We found that GRK2 increased serine phosphorylation of the C-terminus of the β-ENaC subunit. Taken together, these finding suggest that GRK2 increases activity of ENaC by phosphorylating the C-terminus of the β-ENaC rendering the channel insensitive to Nedd4-2.
GRK2 contains a regulator of G protein signaling homology (RH) domain, which selectively interacts with α-subunits of Gq/11 that are released during G protein-coupled receptor activation. We found that this RH domain of GRK2 also plays a role in regulating activity of ENaC. In M1 mouse collecting duct cells, expression of a kinase-dead GRK2 mutant increased activity of ENaC (Lee et al., 2011). Conversely, a GRK2 mutant that lacks the C-terminal RH-domain or a GRK2 mutant that cannot interact with Gαq/11, has no effect on the activity of ENaC. These findings suggest that GRK2 upregulates ENaC as a consequence of the RH domain of GRK2 sequestering the α-subunits of Gq/11. This effect of GRK2 is independent of Nedd4-2 and does not involve inhibition of expression of ENaC in the plasma membrane or proteolytic activation of the channel (Lee et al., 2011). The phosphorylation-independent effect that is mediated by the RH domain of GRK2 provides an additional regulatory mechanism for controlling epithelial Na+ absorption.
Dinudom A, Fotia AB, Lefkowitz RJ, Young JA, Kumar S & Cook DI. (2004) The kinase GRK2 regulates Nedd4/Nedd4-2-dependent control of epithelial Na+ channels. Proceedings of the National Academy of Sciences U.S.A. 101: 11886-11890.
Lee IH, Song SH, Campbell CR, Kumar S, Cook DI & Dinudom A. (2011) Regulation of the epithelial Na+ channel by the RH Domain of G Protein-coupled receptor kinase, GRK2, and Gαq/11. Journal of Biological Chemistry 286: 19259-19269.