Presenter: Norma L. Houston
Advisor(s): Rebecca S. Boston
Author(s): Norma L. Houston and Rebecca S. Boston
Graduate Program: Functional Genomics and Botany

Title: Gene Expression and Phylogenetic Relationships Reveal a Complex Protein Disulfide Isomerase Family in Plants

Abstract: In maize, mutations that lead to the accumulation of misfolded storage proteins can cause dramatic phenotypic effects, such as soft, starchy kernels that are unsuitable for commercial applications. We used a combined genomic, biochemical and bioinformatics approach to characterize protein disulfide isomerases (PDIs), a class of proteins critical for proper protein folding. To identify plant PDI-like (PDIL) proteins, we performed a genome-wide search of Arabidopsis thaliana and identified 104 candidate proteins, 22 of which group with orthologs of plant PDIs in a well supported phylogenetic clade. Using the Arabidopsis PDIL sequences in bioinformatic searches of other databases, we identified orthologous sets of plant PDIL sequences in rice (19) and maize (22). Phylogenetic analysis resolved the phylogeny into 10 classes. Five classes had two structural domains typically found in PDIL proteins in other higher eukaryotes while the remaining five classes (VI-X) had a single domain. RNA profiling of the maize PDILs (ZmPDILs) showed marked differences within and across the PDIL classes. The major PDI (ZmPDIL1-1), the class V PDIL (ZmPDIL2-3) and the class VI maize PDIL (ZmPDIL5-1) were up-regulated during a cellular reaction to the presence of unfolded proteins, the endoplasmic reticulum (ER) stress response. Subcellular fractionation and immunoblot analysis revealed that ZmPDIL1-1 but not ZmPDIL5-1 was localized to intracellular membrane fractions that included the ER. Protein isomerase activity was detected in ZmPDIL1-1 but not ZmPDIl5-1. ER-stress induction in the absence of ER localization and isomerase activity is suggestive of an unusual role for the PDIL5-1 family member. Further characterization of the expression and enzymatic activity of the maize PDIL proteins will improve our understanding of their contribution to seed quality. In addition, our work may contribute new models for cellular responses to protein misfolding, a problem not only in seeds but in many diseases such as Alzheimer's and Creutzfeldt-Jakob diseases.