A RAPID INCREASE IN THE LEVEL OF BINDING-PROTEIN (BIP) IS ACCOMPANIEDBY SYNTHESIS AND DEGRADATION OF STORAGE PROTEINS IN PUMPKIN COTYLEDONS

The binding protein (BiP) has been implicated in cotranslational foldi ng of nascent polypeptides, and in the recognition and disposal of abe rrant polypeptides. To elucidate the involvement of BiP in the biosynt hesis of vacuolar proteins, we have characterized the protein in pumpk in cotyledons during seed maturation and seedling growth. Isolated mic rosomes from maturing pumpkin cotyledons contained a significant amoun t of BiP, protein-disulfide isomerase and calreticulin. We have purifi ed a 70-kDa protein; sequences of the N-terminus and internal fragment s of this protein exhibited a high identity to the sequence of soybean BiP. Immunoblot analysis with specific antibodies raised against the purified BiP showed that the amount of BiP in a cotyledon increased ma rkedly at the middle stages and then decreased. The increase was accom panied by the synthesis of storage proteins and the development of the endoplasmic reticulum in the cotyledons at the middle stage of seed m aturation. Most of these storage proteins degraded dramatically betwee n 2 and 5 days after seed germination, and the degradation was also ac companied by a rapid increase in the level of BiP. Subcellular fractio nation of the 4-day-old cotyledons showed a high accumulation of BiP i n the endoplasmic reticulum. It is possible that BiP might be involved in the synthesis of seed storage proteins during maturation and in th e synthesis of hydrolytic enzymes responsible for the degradation of t he storage proteins during seed germination.