MOLECULAR CHARACTERIZATION OF PROTEINS IN PROTEIN-BODY MEMBRANE THAT DISAPPEAR MOST RAPIDLY DURING TRANSFORMATION OF PROTEIN BODIES INTO VACUOLES

During the post-germination growth of seeds, protein bodies fuse with one another and are converted to a central vacuole. To investigate this transition, protein-body membranes from dry seeds of pumpkin (Cucurbita sp.) were prepared and their protein components characterized. Five major proteins (designated MP23, MP27, MP28, MP32 and MP73) were detected in the protein-body membranes. A cDNA clone encoding both MP27 and MP32 has been isolated. The deduced precursor polypeptide was composed of a hydrophobic signal sequence, MP27 and MP32, in that order. A putative site of cleavage between MP27 and MP32 was located on the COOH-terminal side of asparagine 278, an indication that the post-translational cleavage may occur by the action of a vacuolar processing enzyme that converts proprotein precursors of seed proteins into the mature forms. Immunoelectron microscopic analysis showed that MP27 and MP32 were associated with protein-body membrane of dry pumpkin seeds. Among the five membrane proteins, MP27 and MP32 disappeared most rapidly during seedling growth. The degradation of MP27 and MP32 starts just after seed germination and proceeds in parallel with the transformation of the protein bodies into a vacuole.