Understanding molecular recognition by G protein βγ subunits on the path to pharmacological targeting

Abstract

Heterotrimeric G proteins, composed of Gα and Gβγ subunits, transduce extracellular signals via G-protein-coupled receptors to modulate many important intracellular responses. The Gβγ subunits hold a central position in this signaling system and have been implicated in multiple aspects of physiology and the pathophysiology of disease. The Gβ subunit belongs to a large family of WD40 repeat proteins with a circular β-bladed propeller structure. This structure allows Gβγ to interact with a broad range of proteins to play diverse roles. How Gβγ interacts with and regulates such a wide variety of partners yet maintains specificity is an interesting problem in protein-protein molecular recognition in signal transduction, where signal transfer by proteins is often driven by modular conserved recognition motifs. Evidence has accumulated that one mechanism for Gβγ multitarget recognition is through an intrinsically flexible protein surface or "hot spot" that accommodates multiple modes of binding. Because each target has a unique recognition mode for Gβγ subunits, it suggests that these interactions could be selectively manipulated with small molecules, which could have significant therapeutic potential.

Fig. 1.

Gβγ crystal structures. A, Gβ₁γ₁ (PDB code: 1TBG) (Sondek et al., 1996). B, Gβ₁γ₁-phosducin (PDB code: 2TRC) (Gaudet et al., 1996). C, Gα_t/iβ₁γ₁ heterotrimer (PDB code: 1GOT) (Lambright et al., 1996). D, Gβ₁γ₂-SIGK peptide (PDB code: 1XHM) (Davis et al., 2005). E, Gβ₁γ₂-GRK2 (PDB code: 1OMW) (Lodowski et al., 2003). Gβ subunits are all colored in aquamarine; Gγ subunits are all colored in green; Gα_t/i is colored in magenta with bound GDP shown as space fill in blue; phosducin is colored in marine; GRK2 is colored in yellow; and SIGK peptide is colored in salmon.

Fig. 2.

Binding of M201 to the Gβγ hot spot. M201 (NSC201400) is depicted in yellow. Gβ is in blue with some of the key amino acids in the hot spot shown in Corey-Pauling-Koltun form and labeled.

Fig. 3.

Therapeutic targets for Gβγ inhibitors.

Fig. 4.

Gβγ inhibitors bias the action of μ opioid receptor agonists. M119/gallein potentiates the analgesic potency of morphine in vivo. It blocks PLC activation but not calcium channel regulation in vitro. We propose that Gβγ inhibitors bias the action of morphine by blocking a hyperalgesic pathway, dependent on PLC activation, downstream of the μ-opioid receptor without blocking Gα subunit signaling or Gβγ-dependent Ca²⁺ channel or K⁺ channel regulation, thus potentiating opioid analgesia.