Background: Vitamin D is a key nutrient in bone health and the vitamin D status of individuals with regular exposure to solar or artifi cial ultraviolet B (UVB) radiation is generally superior to those with limited exposure.

Polymorphisms in G protein receptor kinase 4γ (GRK4γ) have been implicated in hypertension via desensitizing Gs‐coupled receptors, such as the β2‐adrenergic receptor (β2‐AR). However, the molecular mechanism initiating phosphorylation and desensitization is unknown. Previous data demonstrate that GRK4γ interacts with inactive Gαs; thus, we hypothesized that the interaction of GRK4γ with Gαs is required for proper desensitization of the β2‐AR. To test this, we identified Gαs constructs that do not fully interact with GRK4γ and used these constructs in functional studies utilizing GnasE2‐/E2‐ mouse embryonic fibroblasts (MEFs), which are functionally Gαs knockouts. The data suggest that GRK4γ binds to Gαs; however, the precise area and amino acids involved in binding has yet to be identified. Moreover, the data confirm that expression of Gαs in GnasE2‐/E2‐ MEFs restores isoproterenol‐mediated generation of cAMP, which is inhibited by GRK4γ; however, when GRK4γ does not bind to Gαs it cannot inhibit β2‐ARs. In conclusion, the data suggests that GRK4γ binds to inactive Gαs positioning it near the receptor, and upon activation of the receptor Gαs dissociates from the receptor, Gβγ, and GRK4γ, resulting in signaling; then GRK4γ, which has previously been reported to be constitutively active, has access to the receptor and phosphorylates and desensitizes the receptor. This work was supported by the AHAF and is supported by a VISN 15 grant.

GRK4γ is constitutively active, and polymorphisms of GRK4γ are linked to clinical hypertension and can induce experimental hypertension. Recent studies demonstrate that GRK4γ interacts with inactive Gαs without interfering with the G βγ complex. To better understand this interaction and GRK4γ function, we investigated the site of interaction between Gαs and GRK4γ utilizing chimeric Gαs/q and Gαs/i1 subunits (only Gαs and Gα13 interact with GRK4γ). Chimeric Gαs/q suggested that GRK4γ interacts with the amino‐terminal portion of Gαs, which was confirmed with the chimeric Gαs/i1. These data indicate that GRK4γ interacts with amino acids 60 ‐ 138 in the helical domain of Gαs. Sequence alignment between the Gα subunits suggested 3 specific points of interaction in the αB helix (Q111, D115, and S119), but after site directed mutagenesis GRK4γ still bound to Gαs. Therefore, the interaction involves multiple amino acids. Since the structure of the helical domain of Gαs that interacts with GRK4γ resembles a hook, much of the helical domain may be involved in interacting with GRK4γ. Binding to the alpha helical domain of Gαs explains how GRK4γ can bind to inactive Gαs and not displace Gβγ as well as account for its constitutive activity. Support was provided by American Health Assistance Foundation and Department of Veterans affairs VISN 15.