Lokits, AD; Indrischek, H; Meiler, J; Hamm, HE; Stadler, PF

Background: Heterotrimeric G proteins are fundamental signaling proteins composed of three subunits, G alpha and a G beta gamma dimer. The role of Ga as a molecular switch is critical for transmitting and amplifying intracellular signaling cascades initiated by an activated G protein Coupled Receptor (GPCR). Despite their biochemical and therapeutic importance, the study of G protein evolution has been limited to the scope of a few model organisms. Furthermore, of the five primary Ga subfamilies, the underlying gene structure of only two families has been thoroughly investigated outside of Mammalia evolution. Therefore our understanding of Ga emergence and evolution across phylogeny remains incomplete. Results: We have computationally identified the presence and absence of every Ga gene (GNA-) across all major branches of Deuterostomia and evaluated the conservation of the underlying exon-intron structures across these phylogenetic groups. We provide evidence of mutually exclusive exon inclusion through alternative splicing in specific lineages. Variations of splice site conservation and isoforms were found for several paralogs which coincide with conserved, putative motifs of DNA-/ RNA-binding proteins. In addition to our curated gene annotations, within Primates, we identified 15 retrotranspositions, many of which have undergone pseudogenization. Most importantly, we find numerous deviations from previous findings regarding the presence and absence of individual GNA-genes, nuanced differences in phyla-specific gene copy numbers, novel paralog duplications and subsequent intron gain and loss events. Conclusions: Our curated annotations allow us to draw more accurate inferences regarding the emergence of all Ga family members across Metazoa and to present a new, updated theory of Ga evolution. Leveraging this, our results are critical for gaining new insights into the co-evolution of the Ga subunit and its many protein binding partners, especially therapeutically relevant G protein - GPCR signaling pathways which radiated in Vertebrata evolution.