BiP, HSP70, NDK and PDI in wheat endosperm. I. Accumulation of mRNA and protein during grain development

Abstract

Biosynthesis and accumulation of seed storage proteins such as the wheat glutens depend on the activity of a variety of other proteins, including chaperones and foldases. cDNA probes and antibodies to two chaperone proteins and a foldase were used to follow mRNA and protein accumulation in developing grains of wheat (Triticum aestivum, cvs Cheyenne and Butte). Endosperm was separated from other grain components and protein accumulation was analyzed on a per mg fresh weight basis. The ER resident chaperone BiP (binding protein) and foldase PDI (protein disulfide isomerase) accumulated to maximal levels in the middle stage of endosperm development, a period of rapid cell expansion and storage protein accumulation, whereas levels of a cytosolic chaperone, HSP70, remained relatively constant throughout grain development. In contrast, nucleoside diphosphate kinase (NDK), a cytosolic enzyme needed for synthesis of nucleoside triphosphates, accumulated early in endosperm development during the period of nuclear division and cell formation. When analyzed as a fraction of total protein the relative abundance of all four proteins peaked early in grain development and then declined. Accumulation of mRNA for the four proteins also peaked early in grain development. Although BiP and PDI formed a declining percentage of total protein as storage protein accumulated, their pattern of accumulation was compatible with a proposed role as catalysts for storage protein folding and accumulation in the ER.