A Rapid Increase in the Level of Binding Protein (BiP) is Accompanied by Synthesis and Degradation of Storage Proteins in Pumpkin Cotyledons

The binding protein (BiP) has been implicated in cotranslationalfolding of nascent polypeptides, and in the recognition anddisposal of aberrant polypeptides. To elucidate the involvementof BiP in the biosynthesis of vacuolar proteins, we have characterizedthe protein in pumpkin cotyledons during seed maturation andseedling growth. Isolated microsomes from maturing pumpkin cotyledonscontained a significant amount of BiP, protein-disulfide isomeraseand calreticulin. We have purified a 70-kDa protein; sequencesof the N-terminus and internal fragments of this protein exhibiteda high identity to the sequence of soybean BiP. Immunoblot analysiswith specific antibodies raised against the purified BiP showedthat the amount of BiP in a cotyledon increased markedly atthe middle stages and then decreased. The increase was accompaniedby the synthesis of storage proteins and the development ofthe endoplasmic reticulum in the cotyledons at the middle stageof seed maturation. Most of these storage proteins degradeddramatically between 2 and 5 days after seed germination, andthe degradation was also accompanied by a rapid increase inthe level of BiP. Subcellular fractionation of the 4-day-oldcotyledons showed a high accumulation of BiP in the endoplasmicreticulum. It is possible that BiP might be involved in thesynthesis of seed storage proteins during maturation and inthe synthesis of hydrolytic enzymes responsible for the degradationof the storage proteins during seed germination. (Received September 18, 1996; Accepted January 8, 1997)