Ca21 Binding Protein Calreticulin in Chlamydomonas Reinhardtii (chlorophyta): Biochemical Characterization, Differential Expression during Sexual Reproduction, and Phylogenetic Analysis1

The occurrence of calreticulin, the main Ca21 binding protein in the endoplasmic reticulum of eukaryotic cells, was investigated in the unicellular green alga Chlamydomonas reinhardtii Dangeard. The biochemical characterization of a diethylaminoethyl purified extract highlighted the presence, on SDSPAGE, of a 55-kDa protein that stained blue with the Stains All dye, a diagnostic feature of acidic Ca21 binding proteins. Immunoblot analyses revealed a strong cross-reaction of the Chlamydomonas reinhardtii protein with antibodies to plant calreticulins and the endoplasmic reticulum retention signal HDEL. Furthermore, the 55-kDa protein bound [45Ca21] and had an acidic isoelectric point (pI 5 4.9) but was neither glycosylated nor phosphorylated. N-terminal sequencing revealed strong amino acid sequence similarity to calreticulin from other sources. The presence of calreticulin in Chlamydomonas reinhardtii suggested that an endoplasmic reticulum Ca21 buffering mechanism was present in this unicellular chlorophyte. The data suggest an early origin and high conservation of endoplasmic-reticulummediated Ca21 functions in eukaryotes, whereby specific posttranslational modifications of the protein