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

Phaseolin is a trimeric glycoprotein that accumulates in the protein storage vacuoles of common bean (Phaseolus vulgaris L.) seeds. The resistance 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 linked to the assembly of phaseolin subunits into trimers, a rapid and efficient process occurring in the endoplasmic reticulum. In bean cotyledonary cells phaseolin trimerization is likely to be assisted by cellular factors and occurs also in the presence of inhibitors of N-linked glycosylation. A mechanism devoted to the retention of unassembled subunits and of assembly intermediates in an early compartment of the secretory pathway would guarantee that only the protease-resistant form of the protein is transported to the storage vacuoles, contributing to the overall efficiency of storage protein accumulation.