Intracellular sites of synthesis and processing of lectin in developing pea cotyledons

The biosynthesis and processing of pea lectin was studied in developing pea cotyledons by a combination of pulse-chase experiments using 14C-aminoacids and subcellular fractionation of the homogenates. Lectin polypeptides were isolated on immunoaffinity gels and fractionated on sodium dodecyl sulfate-polyacrylamide gels followed by fluorography. Whether made in vivo or in an in vitro chain completion system containing polysomes still attached to microsomal membranes, newly synthesized lectin had an Mr of 23,000. In vivo labeling showed that radioactive lectin first associates with the rough endoplasmic reticulum and is then sequestered into the lumen of the endoplasmic reticulum. Pulse-chase experiments showed that it is chased out of the endoplasmic reticulum considerably more slowly than the storage proteins and accumulates in the protein bodies, initially as a polypeptide of Mr = 23,000. This pro-lectin is processed in the protein bodies to its mature form with polypeptides of Mr = 17,000 and 6,000. Pea lectin is synthesized throughout the protein accumulation phase of seed formation with maximum levels of synthesis and of mRNA approximately half-way through this period.