Surface potential representation of Gβγ with SIGK (blue ribbon) bound at the “hot spot”. Blue areas are positively charged, red areas are negatively charged and white areas are neutral.

(A) Domain organization of PLCδ and PLCβ. Pleckstrin homology domain (PH), EF hand domain (EF), catalytic domain (X and Y), C2 domain (C2). (B) Ribbon representation of PLCβ2 (from coordinates 2FJU) with domains color coded as for A: in turquoise, the helical region of the PLCβ2 catalytic domain (574–583) found to interact with Gβγ; in dark blue, the X-Y linker domain that caps the enzyme active site; in purple, a region of the PH domain important for Gβγ-dependent PLC activation; in CPK spacefill, the catalytic histidine required for PIP₂ hydrolysis.

(A) Domain organization of the R7 family of RGS proteins. DEP is the Disheveled, EGL-10, Pleckstrin homology domain, R7H is the R7 homology domain, ggl is the G protein γ like domain, and RGS is the RGS homology domain. (B) Depiction of the interactions of an R7 protein with Gβ with the ggl domain interacting with Gβ₅ instead of Gγ and the DEP domain potentially interacting with the Gα subunit binding site on Gβ. (C) Model depicting palmitoylated R7 binding protein bound to the DEP domain of R7. Evidence suggests that this binding could alter DEP domain interactions with Gβ₅ to expose binding surfaces on Gβ₅. The palmitoylated R7BP could then direct binding to the plasma membrane.

Models for participation of Gβγ subunits in regulation of nucleotide exchange. (A) Lever arm model for Gβγ dependent activation of Gα subunit nucleotide exchange. In this model R* interaction with the N-terminus of the Gα subunit and the C-terminus of Gγ results in Gβγ acting as a lever to pull open the nucleotide binding site on Gα to enhance the rate of GDP release. Critical interactions at the Gα switch II/Gβγ interface (GβD228 and GαK206) are required for Gβ to pull on Gα switch II to open the nucleotide binding pocket. (B) Gear shift model for Gβγ-dependent activation of nucleotide exchange. Here the receptor causes the Gβγ subunits to move closer to the Gα subunit. This results in a potential physical interaction between the N-terminal helical Gβγ coiled-coil extension and the helical domain of Gα, leading to opening of the nucleotide binding pocket and release of GDP.

Models for “breathing” of Gα/βγ interfaces and non-receptor dependent activation mechanisms. (A) In the center is a ribbon diagram depicting Gα binding to Gβγ showing the two components of a bivalent interaction of Gα with Gβγ, the Gα N-terminal helix interaction with the side of blade one of the Gβ β-propeller and the Gα switch II interaction with the top of the Gβ β-propeller (see Fig 1.). In this model either of these two interfaces can open and close in rapid equilibrium without subunit dissociation. Only when both contacts are broken simultaneously can subunit dissociation occur. (B) SIGK-dependent subunit dissociation. In this model when the Gα switch II Gβ interfaces open up, SIGK can bind and prevent closure of this interface, resulting in an enhanced rate of subunit dissociation. (C) AGS8-dependent G protein activation. Similar to, B., when the Gα switch II Gβ interface opens AGS8 binds, but since AGS8 can bind to Gα and Gβ, the bivalent interaction of Gα with the complex is maintained and subunit dissociation does not occur.

(A) The G protein cycle. The Gα subunit bound to GDP interacts with the Gβ subunit with two contacts involving the N-terminal domain and the GTP binding domain. This is the inactive resting state. This complex is a substrate for the activated GPCR (R*) which catalyzes an increase in the dissociation rate of GDP from the Gα subunit leading to the nucleotide free Gαβγ complex. This complex is very short lived in the cell where high concentrations of GTP in the cell bind to the empty nucleotide binding site to drive a conformational change in the Gα subunit. This conformation change leads to the active GαGTP subunit, perhaps separated from signaling competent free Gβγ subunits. The Gα subunit has the intrinsic capacity to hydrolyze GTP to GDP, allowing reassembly with Gβγ to return to the resting state. Regulators of G protein signaling (RGS proteins) can bind to Gα and enhance the rate of GTP hydrolysis. Pertussis Toxin (PTX) modifies the G protein α subunit and prevents interactions of the GαGDPβγ heterotrimer with R*. Also shown are myristoyl and isoprenoid lipid groups at the Gα N-terminus and Gγ C-terminus respectively. (B) Ribbon diagram representing the three dimensional crystal structure of the Gβγ subunits. The blades of the propeller are numbered as in Wall et al. [11]. The Gβ N-terminal helix is in red and forms a coiled coil interaction with the Gγ subunit in blue. (C) Ribbon diagram of the G protein heterotrimer with the Gβγ subunit rotated 90° relative to B. The Gα N-terminus is represented as a yellow helix and the Gα Switch II region is in dark blue.