Genomic sequence of a calnexin homolog from Arabidopsis thaliana

• Published in: Plant physiology (Bethesda) 1994-12, Vol.106 (4), p.1691-1691
• Main Authors: Boyce, J.M. (University of Oxford, Oxford, UK.), Coates, D, Fricker, M.D, Evans, D.E
• Format: Article
• Language: English
• Subjects: ADN; AMINO ACID SEQUENCES; Analytical, structural and metabolic biochemistry; Animals; Arabidopsis - genetics; ARABIDOPSIS THALIANA; Base Sequence; Binding and carrier proteins; Biological and medical sciences; CALCIO; CALCIUM; Calcium-Binding Proteins - genetics; Calnexin; calreticulin; CHEMICAL COMPOSITION; Complementary DNA; COMPOSICION QUIMICA; COMPOSITION CHIMIQUE; Databases, Factual; DNA; ENREGISTREMENT; Fundamental and applied biological sciences. Psychology; genbank/u08315; GENE; GENES; Genes, Plant; Genes. Genome; Genomics; Membrane Proteins - genetics; Molecular and cellular biology; Molecular genetics; Molecular Sequence Data; NUCLEOTIDE SEQUENCE; Plant Gene Register; PROTEINAS; PROTEINE; PROTEINS; REGISTRATION; REGISTRO; RETICULO; RETICULUM; SECUENCIA NUCLEICA; Sequence Homology, Nucleic Acid; SEQUENCE NUCLEIQUE
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.106.4.1691

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.