Stabilization of interdomain interactions in G protein α subunits as a determinant of Gαi subtype signaling specificity

Highly homologous members of the Gαi family, Gαi1-3, have distinct tissue distributions and physiological functions, yet their biochemical and functional properties are very similar. We recently identified PDZ-RhoGEF (PRG) as a novel Gαi1 effector that is poorly activated by Gαi2. In a proteomic proximity labeling screen we observed a strong preference for Gαi1 relative to Gαi2 with respect to engagement of a broad range of potential targets. We investigated the mechanistic basis for this selectivity using PRG as a representative target. Substitution of either the helical domain (HD) from Gαi1 into Gαi2 or substitution of a single amino acid, A230 in Gαi2 with the corresponding D in Gαi1, largely rescues PRG activation and interactions with other potential Gαi targets. Molecular dynamics simulations combined with Bayesian network models revealed that in the GTP bound state, separation at the HD-Ras-like domain (RLD) interface is more pronounced in Gαi2 than Gαi1. Mutation of A230 to D in Gαi2 stabilizes HD-RLD interactions via ionic interactions with R145 in the HD which in turn modify the conformation of Switch III. These data support a model where D229 in Gαi1 interacts with R144 and stabilizes a network of interactions between HD and RLD to promote protein target recognition. The corresponding A230 in Gαi2 is unable to stabilize this network leading to an overall lower efficacy with respect to target interactions. This study reveals distinct mechanistic properties that could underly differential biological and physiological consequences of activation of Gαi1 or Gαi2 by G protein-coupled receptors.

Distinct Interaction Modes for the Eukaryotic RNA Polymerase Alpha-like Subunits

Eukaryotic DNA-dependent RNA polymerases (Pols I-III) encode two distinct alpha-like heterodimers where one is shared between Pols I and III, and the other is unique to Pol II. Human alpha-like subunit mutations are associated with several diseases including Treacher Collins Syndrome (TCS), 4H leukodystrophy, and primary ovarian sufficiency. Yeast is commonly used to model human disease mutations, yet it remains unclear whether the alpha-like subunit interactions are functionally similar between yeast and human homologs. To examine this, we mutated several regions of the yeast and human small alpha-like subunits and used biochemical and genetic assays to establish the regions and residues required for heterodimerization with their corresponding large alpha-like subunits. Here we show that different regions of the small alpha-like subunits serve differential roles in heterodimerization, in a polymerase- and species-specific manner. We found that the small human alpha-like subunits are more sensitive to mutations, including a "humanized" yeast that we used to characterize the molecular consequence of the TCS-causingPOLR1D G52E mutation. These findings help explain why some alpha subunit associated disease mutations have little to no effect when made in their yeast orthologs and offer a better yeast model to assess the molecular basis of POLR1D associated disease mutations.

Glycine receptor α3 and α2 subunits mediate tonic and exogenous agonist-induced currents in forebrain

Neuronal inhibition can occur via synaptic mechanisms or through tonic activation of extrasynaptic receptors. In spinal cord, glycine mediates synaptic inhibition through the activation of heteromeric glycine receptors (GlyRs) composed primarily of α1 and β subunits. Inhibitory GlyRs are also found throughout the brain, where GlyR α2 and α3 subunit expression exceeds that of α1, particularly in forebrain structures, and coassembly of these α subunits with the β subunit appears to occur to a lesser extent than in spinal cord. Here, we analyzed GlyR currents in several regions of the adolescent mouse forebrain (striatum, prefrontal cortex, hippocampus, amygdala, and bed nucleus of the stria terminalis). Our results show ubiquitous expression of GlyRs that mediate large-amplitude currents in response to exogenously applied glycine in these forebrain structures. Additionally, tonic inward currents were also detected, but only in the striatum, hippocampus, and prefrontal cortex (PFC). These tonic currents were sensitive to both strychnine and picrotoxin, indicating that they are mediated by extrasynaptic homomeric GlyRs. Recordings from mice deficient in the GlyR α3 subunit (Glra3^-/-) revealed a lack of tonic GlyR currents in the striatum and the PFC. In Glra2^-/Y animals, GlyR tonic currents were preserved; however, the amplitudes of current responses to exogenous glycine were significantly reduced. We conclude that functional α2 and α3 GlyRs are present in various regions of the forebrain and that α3 GlyRs specifically participate in tonic inhibition in the striatum and PFC. Our findings suggest roles for glycine in regulating neuronal excitability in the forebrain.