Molecular cloning and characterization of two soybean protein disulfide isomerases as molecular chaperones for seed storage proteins

Protein disulfide isomerase family proteins play important roles in the folding of nascent polypeptides and the formation of disulfide bonds in the endoplasmic reticulum. In this study, we cloned two similar protein disulfide isomerase family genes from soybean leaf (Glycine max L. Merrill. cv Jack). The cDNAs encode proteins of 525 and 551 amino acids, named GmPDIL-1 and GmPDIL-2, respectively. Recombinant versions of GmPDIL-1 and GmPDIL-2 expressed in Escherichia coli exhibited oxidative refolding activity for denatured RNaseA. Genomic sequences of both GmPDIL-1 and GmPDIL-2 were cloned and sequenced. The comparison of soybean genomic sequences with those of Arabidopsis, rice and wheat showed impressive conservation of exon-intron structure across plant species. The promoter sequences of GmPDIL-1 apparently contain a cis-acting regulatory element functionally linked to unfolded protein response. GmPDIL-1, but not GmPDIL-2, expression was induced under endoplasmic reticulum-stress conditions. GmPDIL-1 and GmPDIL-2 promoters contain some predicted regulatory motifs for seed-specific expression. Both proteins were ubiquitously expressed in soybean tissues, including cotyledon, and localized to the endoplasmic reticulum. Data from coimmunoprecipitation experiments suggested that GmPDIL-1 and GmPDIL-2 associate with proglycinin, a precursor of the seed storage protein glycinin, and the alpha'-subunit of beta-conglycinin, a seed storage protein found in cotyledon cells under conditions that disrupt the folding of glycinin or beta-conglycinin, suggesting that GmPDIL-1 and GmPDIL-2 are involved in the proper folding or quality control of such storage proteins as molecular chaperones.