Identification and characterization of endoplasmic reticulum-associated degradation proteins differentially affected by endoplasmic reticulum stress.

The disposal of misfolded proteins from the lumen of the endoplasmic reticulum ( ER) is one of the quality control mechanisms present in the protein secretory pathway. Through ER- associated degradation, misfolded substrates are targeted to the cytosol where they are degraded by the proteasome. We have identified four maize ( Zea mays) Der1- like genes ( Zm Derlins) that encode homologs of Der1p, a yeast ( Saccharomyces cerevisiae) protein implicated in ER- associated degradation. Zm Derlins are capable of functionally complementing a yeast Der1 deletion mutant. Such complementation indicates that the Der1p function is conserved among species. Zm Derlin genes are expressed at low levels throughout the plant, but appear prevalent in tissues with high activity of secretory protein accumulation, including developing endosperm cells. Expression of three of the four Zm Derlin genes increases during ER stress, with Zm Derlin1- 1 showing the strongest induction. Subcellular fractionation experiments localized Zm Derlin proteins to the membrane fraction of microsomes. In maize endosperm, Zm Derlin proteins were found primarily associated with ER- derived protein bodies regardless of the presence of an ER stress response.