Primary structure and characterization of an Arabidopsis thaliana calnexin-like protein

A cDNA clone (pTE-83) encoding a protein (CNX1p) related to the microsomal Ca(2+)-binding protein, calnexin, was isolated from an Arabidopsis thaliana expression library. Southern and Northern hybridization indicated that CNX1 is a single-copy gene encoding a message of 1900 nucleotides. The open reading frame encodes a polypeptide with 530 amino acids, a molecular mass of 60.5 kDa, and overall 48% identity to dog calnexin. Both animal calnexin and CNX1p contain a large luminal domain followed by a single potential membrane-spanning domain near the C terminus and a small C-terminal domain exposed to the cytoplasm. The in vitro translation product from the cloned cDNA yielded a polypeptide of 67 kDa that was co-translationally imported into dog microsomes and processed to a 64-kDa product. Antibodies generated against the C-terminal half of the protein cross-react with an identically sized protein present in the microsomal fraction from Arabidopsis. Both the imported and native proteins are cleaved by trypsin to a 59-kDa product indicating that the gene product was indeed correctly processed and translocated into dog microsomes and that the membrane topology of CNX1p resembles that of dog calnexin. The presence of a calnexin-like protein within the plant kingdom indicates that this protein is widespread and involved in processes fundamental to all eukaryotes.