Ca2+ binding protein calreticulin in Chlamydomonas reinhardtii (Chlorophyta): Biochemical characterization differential expression during sexual reproduction, and phylogenetic analysis

The occurrence of calreticulin, the main Ca2+ binding protein in the endopl asmic reticulum of eukaryotic cells, was investigated in the unicellular gr een alga Chlamydomonas reinhardtii Dangeard. The biochemical characterizati on of a diethylaminoethyl purified extract highlighted the presence, on SDS -PAGE, of a 55-kDa protein that stained blue with the Stains All dye, a dia gnostic feature of acidic Ca2+ binding proteins. Immunoblot analyses reveal ed a strong cross-reaction of the Chlamydomonas reinhardtii protein with an tibodies to plant calreticulins and the endoplasmic reticulum retention sig nal HDEL. Furthermore, the 55-kDa protein bound [Ca-45(2+)] and had an acid ic isoelectric point (pI = 4.9) but was neither glycosylated nor phosphoryl ated. N-terminal sequencing revealed strong amino acid sequence similarity to calreticulin from other sources. The presence of calreticulin in Chlamyd omonas reinhardtii suggested that an endoplasmic reticulum Ca2+ buffering m echanism was present in this unicellular chlorophyte. The data suggest an e arly origin and high conservation of endoplasmic-reticulum-mediated Ca2+ fu nctions in eukaryotes, whereby specific posttranslational modifications of the protein have been specifically acquired in different Lineages of photos ynthetic eukaryotes. Moreover, northern and western blot analysis experimen ts showed a regulation of calreticulin expression during Chlamydomonas sexu al reproduction with a high abundance of calreticulin mRNA and protein in r eproductive cells.