Tracing the Evolutionary History of Inositol, 1, 4, 5-Trisphosphate Receptor: Insights from Analyses of Capsaspora owczarzaki Ca2+ Release Channel Orthologs

Cellular Ca2+ homeostasis is tightly regulated and is pivotal to life. Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) are the major ion channels that regulate Ca2+ release from intracellular stores. Although these channels have been extensively investigated inmulticellular organisms, an appreciation of their evolution and the biology of orthologs in unicellular organisms is largely lacking. Extensive phylogenetic analyses reveal that the IP3R gene superfamily is ancient and diverged into two subfamilies, IP3R-A and IP3R-B/RyR, at the dawn of Opisthokonta. IP3R-B/RyR further diversified into IP3R-B and RyR at the stem of Filozoa. Subsequent evolution and speciation of Holozoa is associated with duplication of IP3R-A and RyR genes, and loss of IP3R-B in the vertebrate lineages. To gain insight into the properties of IP3R important for the challenges of multicellularity, the IP3R-A and IP3R-B family orthologs were cloned from Capsaspora owczarzaki, a close unicellular relative to Metazoa (designated as CO. IP3R-A and CO. IP3RB). Both proteins were targeted to the endoplasmic reticulum. However, CO. IP3R-A, but strikingly not CO. IP3R-B, bound IP3, exhibited robust Ca2+ release activity and associated with mammalian IP(3)Rs. These data indicate strongly that CO. IP3R-A as an exemplar of ancestral IP3 R-A orthologs forms bona fide IP3-gated channels. Notably, however, CO. IP3R-A appears not to be regulated by Ca2+, ATP or Protein kinase A-phosphorylation. Collectively, our findings explore the origin, conservation, and diversification of IP3R gene families and provide insight into the functionality of ancestral IP(3)Rs and the added specialization of these proteins in Metazoa.