Abstract
Calnexin is a membrane-bound protein of the ER in animal cells (Wada et al., 1991). It shows considerable similarity to the major calcium-sequestering protein of the ER lumen, calreticulin, with two calcium-binding regions--a high-affinity, low-capacity region in the ER lumen and a low-affinity, high-capacity region in the cytoplasm. The protein is postulated to act as a calcium-regulated chaperone during protein maturation (Ou et al., 1993). We have isolated a genomic sequence showing significant homology to the animal gene over the predicted coding sequence (Table I). A partial cDNA from Zea mays was isolated from an expression library made from 6-d coleoptiles (Clontech, Palo Alto, CA). The library was screened using a monoclonal antibody raised against a small number of microsomal proteins resulting from a partial purification of plasma membrane Ca2+ ATPase (Briars et al., 1988). The partial cDNA showed sequence homology to the calcium-binding region common to calreticulin and calnexin. The fragment was used to screen a genomic library constructed from Arabidopsis thaliana (cv Larasbonerecta), and a 15-kb fragment was isolated and subcloned and the relevant subfragments were sequenced. The coding region contains five introns, two in the N-terminal region and three in the C-terminal region. The predicted amino acid sequence shows a high level of homology with the animal calnexin, although the terminal highly acidic calcium-binding region is shorter. A cDNA for a putative homolog of calnexin was isolated from A. thaliana (cv Columbia) by Huang et al.(1993); our coding sequence shows 85% identity and 92% similarity determined by FASTA (Wisconsin Genetics Computer Group package); however, the differences are greater than would be expected between cultivars of the same species. A Southern blot probed with DNA from the central calcium-binding region shows multiple bands. This, combined with the sequence heterogeneity, suggests that calnexin belongs to a family of related genes.
Full Text
The Full Text of this article is available as a PDF (97.0 KB).
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Huang L., Franklin A. E., Hoffman N. E. Primary structure and characterization of an Arabidopsis thaliana calnexin-like protein. J Biol Chem. 1993 Mar 25;268(9):6560–6566. [PubMed] [Google Scholar]
- Ou W. J., Cameron P. H., Thomas D. Y., Bergeron J. J. Association of folding intermediates of glycoproteins with calnexin during protein maturation. Nature. 1993 Aug 26;364(6440):771–776. doi: 10.1038/364771a0. [DOI] [PubMed] [Google Scholar]
- Wada I., Rindress D., Cameron P. H., Ou W. J., Doherty J. J., 2nd, Louvard D., Bell A. W., Dignard D., Thomas D. Y., Bergeron J. J. SSR alpha and associated calnexin are major calcium binding proteins of the endoplasmic reticulum membrane. J Biol Chem. 1991 Oct 15;266(29):19599–19610. [PubMed] [Google Scholar]