G protein ß subunits regulate Cav3.3 T-type channel activity and current kinetics via interaction with the Cav3.3 C-terminus.

Ca2+ influx through Cav3.3 T-type channel plays crucial roles in neuronal excitability and is subject to regulation by various signaling molecules. However, our understanding of the partners of Cav3.3 and the related regulatory pathways remains largely limited. To address this quest, we employed the rat Cav3.3 C-terminus as bait in yeast-two-hybrid screenings of a cDNA library, identifying rat Gß2 as an interaction partner. Subsequent assays revealed that the interaction of Gß2 subunit was specific to the Cav3.3 C-terminus. Through systematic dissection of the C-terminus, we pinpointed a 22 amino acid sequence (amino acids 1789-1810) as the Gß2 interaction site. Coexpression studies of rat Cav3.3 with various Gßγ compositions were conducted in HEK-293 cells. Patch clamp recordings revealed that coexpression of Gß2γ2 reduced Cav3.3 current density and accelerated inactivation kinetics. Interestingly, the effects were not unique to Gß2γ2, but were mimicked by Gß2 alone as well as other Gßγ dimers, with similar potencies. Deletion of the Gß2 interaction site abolished the effects of Gß2γ2. Importantly, these Gß2 effects were reproduced in human Cav3.3. Overall, our findings provide evidence that Gß(γ) complexes inhibit Cav3.3 channel activity and accelerate the inactivation kinetics through the Gß interaction with the Cav3.3 C-terminus.

Molecular basis for Gßγ-mediated activation of phosphoinositide 3-kinase γ.

The conversion of phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 3,4,5-triphosphate by phosphoinositide 3-kinase γ (PI3Kγ) is critical for neutrophil chemotaxis and cancer metastasis. PI3Kγ is activated by Gßγ heterodimers released from G protein-coupled receptors responding to extracellular signals. Here we determined cryo-electron microscopy structures of Sus scrofa PI3Kγ-human Gßγ complexes in the presence of substrates/analogs, revealing two Gßγ binding sites: one on the p110γ helical domain and another on the p101 C-terminal domain. Comparison with PI3Kγ alone reveals conformational changes in the kinase domain upon Gßγ binding that are similar to Ras·GTP-induced changes. Assays of variants perturbing the Gßγ binding sites and interdomain contacts altered by Gßγ binding suggest that Gßγ recruits the enzyme to membranes and allosterically regulates activity via both sites. Studies of zebrafish neutrophil migration align with these findings, paving the way for in-depth investigation of Gßγ-mediated activation mechanisms in this enzyme family and drug development for PI3Kγ.

Structure of adenylyl cyclase 5 in complex with Gßγ offers insights into ADCY5-related dyskinesia.

The nine different membrane-anchored adenylyl cyclase isoforms (AC1-9) in mammals are stimulated by the heterotrimeric G protein, Gαs, but their response to Gßγ regulation is isoform specific. In the present study, we report cryo-electron microscope structures of ligand-free AC5 in complex with Gßγ and a dimeric form of AC5 that could be involved in its regulation. Gßγ binds to a coiled-coil domain that links the AC transmembrane region to its catalytic core as well as to a region (C1b) that is known to be a hub for isoform-specific regulation. We confirmed the Gßγ interaction with both purified proteins and cell-based assays. Gain-of-function mutations in AC5 associated with human familial dyskinesia are located at the interface of AC5 with Gßγ and show reduced conditional activation by Gßγ, emphasizing the importance of the observed interaction for motor function in humans. We propose a molecular mechanism wherein Gßγ either prevents dimerization of AC5 or allosterically modulates the coiled-coil domain, and hence the catalytic core. As our mechanistic understanding of how individual AC isoforms are uniquely regulated is limited, studies such as this may provide new avenues for isoform-specific drug development.

Molecular dissection of PI3Kß synergistic activation by receptor tyrosine kinases, GßGγ, and Rho-family GTPases.

Phosphoinositide 3-kinase (PI3K) beta (PI3Kß) is functionally unique in the ability to integrate signals derived from receptor tyrosine kinases (RTKs), G-protein coupled receptors, and Rho-family GTPases. The mechanism by which PI3Kß prioritizes interactions with various membrane-tethered signaling inputs, however, remains unclear. Previous experiments did not determine whether interactions with membrane-tethered proteins primarily control PI3Kß localization versus directly modulate lipid kinase activity. To address this gap in our knowledge, we established an assay to directly visualize how three distinct protein interactions regulate PI3Kß when presented to the kinase in a biologically relevant configuration on supported lipid bilayers. Using single molecule Total Internal Reflection Fluorescence (TIRF) Microscopy, we determined the mechanism controlling PI3Kß membrane localization, prioritization of signaling inputs, and lipid kinase activation. We find that auto-inhibited PI3Kß prioritizes interactions with RTK-derived tyrosine phosphorylated (pY) peptides before engaging either GßGγ or Rac1(GTP). Although pY peptides strongly localize PI3Kß to membranes, stimulation of lipid kinase activity is modest. In the presence of either pY/GßGγ or pY/Rac1(GTP), PI3Kß activity is dramatically enhanced beyond what can be explained by simply increasing membrane localization. Instead, PI3Kß is synergistically activated by pY/GßGγ and pY/Rac1 (GTP) through a mechanism consistent with allosteric regulation.