Bean homologs of the mammalian glucose‐regulated proteins: induction by tunicamycin and interaction with newly synthesized seed storage proteins in the endoplasmic reticulum

Abstract

Treatment of developing bean cotyledons with the inhibitor of N ‐glycosylation tunicamycin enhanced the synthesis of at least two polypeptides with molecular mass 78 kDa and 97 kDa. Pulse‐chase experiments and subcellular fractionation indicated that these are endoplasmic reticulum (ER) residents. The 78 kDa protein is a major component of the ER protein fraction and, by N‐terminal sequencing, was identified as a bean homolog of the mammalian 78 kDa glucose‐regulated protein (GRP78). This is a molecular chaperone that is probably involved in the folding and oligomerization of several animal and yeast proteins in the ER. When newly synthesized storage glyco‐proteins phaseolin, phytohemagglutinin or α‐amylase inhibitor were immunoprecipitated from an ER preparation of tunicamycin‐treated tissue, the GRP78 homolog was always co‐precipitated. Bound GRP78 homolog could be released by ATP treatment. These results suggest that, at least when glycosylation is inhibited, this protein plays a role in the early stages of the synthesis of vacuolar storage proteins. Copyright © 1992, Wiley Blackwell. All rights reserved