Gβγ-mediated activation of protein kinase D exhibits subunit specificity and requires Gβγ-responsive phospholipase Cβ isoforms

BACKGROUND: Protein kinase D (PKD) constitutes a novel family of serine/threonine protein kinases implicated in fundamental biological activities including cell proliferation, survival, migration, and immune responses. Activation of PKD in these cellular activities has been linked to many extracellular signals acting through antigen receptor engagement, receptor tyrosine kinases, as well as G protein-coupled receptors. In the latter case, it is generally believed that the Gα subunits of the G(q) family are highly effective in mediating PKD activation, whereas little is known with regard to the ability of Gβγ dimers and other Gα subunits to stimulate PKD. It has been suggested that the interaction between Gβγ and the PH domain of PKD, or the Gβγ-induced PLCβ/PKC activity is critical for the induction of PKD activation. However, the relative contribution of these two apparently independent events to Gβγ-mediated PKD activation has yet to be addressed. RESULTS: In this report, we demonstrate that among various members in the four G protein families, only the Gα subunits of the G(q) family effectively activate all the three PKD isoforms (PKD1/2/3), while Gα subunits of other G protein families (G(s), G(i), and G(12)) are ineffective. Though the Gα subunits of G(i) family are unable to stimulate PKD, receptors linked to G(i) proteins are capable of triggering PKD activation in cell lines endogenously expressing (HeLa cells and Jurkat T-cells) or exogenously transfected with (HEK293 cells) Gβγ-sensitive PLCβ(2/3) isoforms. This indicates that the G(i)-mediated PKD activation is dependent on the released Gβγ dimers upon stimulation. Further investigation on individual Gβγ combinations (i.e. Gβ(1) with Gγ(1–13)) revealed that, even if they can stimulate the PLCβ activity in a comparable manner, only those Gβ(1)γ dimers with γ(2), γ(3), γ(4), γ(5), γ(7), and γ(10) can serve as effective activators of PKD. We also demonstrated that G(i)-mediated PKD activation is essential for the SDF-1α-induced chemotaxis on Jurkat T-cells. CONCLUSIONS: Our current report illustrates that Gβγ dimers from the G(i) proteins may activate PKD in a PLCβ(2/3)-dependent manner, and the specific identities of Gγ components within Gβγ dimers may determine this stimulatory action.