Accumulation of a fusion protein containing 2S albumin induces novel vesicles in vegetative cells of Arabidopsis.

We have previously reported that precursor-accumulating (PAC) vesicles found exclusively in developing seeds are involved in a transport of seed storage proteins, such as 2S albumin, from the endoplasmic reticulum to protein-storage vacuoles. Here, we constructed chimeric genes that encode fusion proteins consisting of both various lengths of polypeptides derived from pumpkin 2S albumin and a selectable marker enzyme, phosphinothricin acetyltransferase. The chimeric genes were expressed in transgenic Arabidopsis in order to investigate the mechanism of the PAC vesicle formation. A fusion protein expressed by one of the chimeric genes is accumulated as a proprotein-precursor form, and localized in novel vesicles of vegetative cells. The vesicles show distinct features that well much to the PAC vesicles. Despite of the accumulation of the fusion protein, the transgenic Arabidopsis is still sensitive to phosphinothricin. Phosphinothricin acetyltransferase contained in the fusion protein is obviously compartmentalized in the PAC-like vesicles that do not permit the detoxification of this herbicide. These results indicate that the PAC-like vesicle can be induced in vegetative cells by the ectopic expression of the protein that is destined to be compartmentalized into the PAC vesicles.