ASSEMBLY AND INTRACELLULAR-TRANSPORT OF PHASEOLIN, THE MAJOR STORAGE PROTEIN OF PHASEOLUS-VULGARIS L

Phaseolin is a trimeric glycoprotein that accumulates in the protein s torage vacuoles of common bean (Phaseolus vulgaris L.) seeds. The resi stance to proteolytic degradation is likely to be an important feature of native phaseolin, allowing its stable accumulation in the protein storage vacuoles. Acquisition of a protease-resistant structure is lin ked to the assembly of phaseolin subunits into trimers, a rapid and ef ficient process occurring in the endoplasmic reticulum. In bean cotyle donary cells phaseolin trimerization is likely to be assisted by cellu lar factors and occurs also in the presence of inhibitors of N-linked glycosylation. A mechanism devoted to the retention of unassembled sub units and of assembly intermediates in an early compartment of the sec retory pathway would guarantee that only the protease-resistant form o f the protein is transported to the storage vacuoles, contributing to the overall efficiency of storage protein accumulation.