CA super(2+) binding protein calreticulin in Chlamydomonas reinhardtii (Chlorophyta): Biochemical characterization, differential expression during sexual reproduction, and phylogenetic analysis

• Published in: Journal of phycology Vol. 35; no. 6; pp. 1224 - 1232
• Main Authors: Zuppini, A, Barbato, R, Bergantino, E, Dainese, P, Meggio, F, Martin, W, Mariani, P
• Format: Journal Article
• Language: English
• Published: 01.12.1999
• Subjects: calcium-binding protein; calreticulin; Chlamydomonas reinhardtii; Freshwater
• ISSN: 0022-3646

The occurrence of calreticulin, the main Ca super(2+) 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 SDS-PAGE, of a 55-kDa protein that stained blue with the Stains All dye, a diagnostic feature of acidic Ca super(2+) 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 [ super(45)Ca super(2+)] and had an acidic isoelectric point (pI = 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 Ca super(2+) buffering mechanism was present in this unicellular chlorophyte. The data suggest an early origin and high conservation of endoplasmic-reticulum-mediated Ca super(2+) functions in eukaryotes, whereby specific posttranslational modifications of the protein have been specifically acquired in different lineages of photosynthetic eukaryotes. Moreover, northern and western blot analysis experiments showed a regulation of calreticulin expression during Chlamydomonas sexual reproduction with a high abundance of calreticulin mRNA and protein in reproductive cells.