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

摘要

Cellular Ca²⁺ homeostasis is tightly regulated and is pivotal to life. Inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are the major ion channels that regulate Ca²⁺ release from intracellular stores. Although these channels have been extensively investigated in multicellular 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.IP3R-B). 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 IP₃Rs. These data indicate strongly that CO.IP₃R-A as an exemplar of ancestral IP₃R-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 IP₃Rs and the added specialization of these proteins in Metazoa.