The ricinosomes of senescing plant tissue bud from the endoplasmic reticulum

The ricinosome (synonym, precursor protease vesicle) is a novel organelle, found so far exclusively in plant cells. Electron microscopic studies sugge st that it buds off from the endoplasmic reticulum in senescing tissues. Bi ochemical support for this unusual origin now comes from the composition of the purified organelle, which contains large amounts of a 45-kDa cysteine endoprotease precursor with a C-terminal KDEL motif and the endoplasmic ret iculum lumen residents BiP (binding protein) and protein disulfide isomeras e, Western blot analysis, peptide sequencing, and mass spectrometry demonst rate retention of KDEL in the protease preform. Acidification of isolated r icinosomes causes castor bean cysteine endopeptidase activation, with cleav age of the N-terminal propeptide and the C-terminal KDEL motif, We propose that ricinosomes accumulate during senescence by programmed cell death and are activated by release of protons from acidic vacuoles.