Functional Conservation of Calreticulin in Euglena gracilis

Calreticulin is the major high capacity, low affinity Ca 2+ binding protein localized within the endoplasmic reticulum. It functions as a reservoir for triggered release of Ca 2+ by the endoplasmic reticulum and is thus integral to eukaryotic signal transduction pathways involving Ca 2+ as a second messenger. The early branching photosynthetic protist Euglena gracilis is shown to possess calreticulin as its major high capacity Ca 2+ binding protein. The protein was purified, microsequenced and cloned. Like its homologues from higher eukaryotes, calreticulin from Euglena possesses a short signal peptide for endoplasmic reticulum import and the C‐terminal retention signal KDEL, indicating that these components of the eukaryotic protein routing apparatus were functional in their present form prior to divergence of the euglenozoan lineage. A gene phytogeny for calreticulin and calnexin sequences in the context of eukaryotic homologues indicates i) that these Ca 2+ binding endoplasmic reticulum proteins descend from a gene duplication that occurred in the earliest stages of eukaryotic evolution and furthermore iii that Euglenozoa express the calreticulin protein of the kinetoplastid (trypanosomes and their relatives) lineage, rather than that of the eukaryotic chlorophyte which gave rise to Euglena's plastids. Evidence for conservation of endoplasmic reticulum routing and Ca 2+ binding function of calreticulin from Euglena traces the functional history of Ca 2+ second messenger signal transduction pathways deep into eukaryotic evolution.