Disulfide formation in plant storage vacuoles permits assembly of a multimeric lectin.

The ER (endoplasmic reticulum) has long been considered the plant cell compartment within which protein disulfide bond formation occurs. Members of the ER-located PDI (protein disulfide isomerase) family are responsible for oxidizing, reducing and isomerizing disulfide bonds, as well as functioning as chaperones to newly synthesized proteins. In the present study we demonstrate that an abundant 7S lectin of the castor oil seed protein storage vacuole, RCA (Ricinus communis agglutinin 1), is folded in the ER as disulfide bonded A-B dimers in both vegetative cells of tobacco leaf and in castor oil seed endosperm, but that these assemble into (A-B)2 disulfide-bonded tetramers only after Golgi-mediated delivery to the storage vacuoles in the producing endosperm tissue. These observations reveal an alternative and novel site conducive for disulfide bond formation in plant cells.