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First published online August 1, 2008; 10.1105/tpc.108.200810 The Plant Cell 20:2006
A Protein Disulfide Isomerase Plays a Role in Programmed Cell DeathScience Editor kfarquharson{at}aspb.org
Programmed cell death (PCD), the tightly regulated self-destruction of specific cells, is crucial for the development and survival of plants (Greenberg, 1996
Prompted by the observation that protein disulfide isomerase (PDI) is present in castor bean ricinosomes, ER-derived organelles that proliferate in cells undergoing PCD (Schmid et al., 2001 Yeast two-hybrid analysis identified three proteins that interacted specifically with PDI5, all of which were Cys proteases (RD21, CP43, and CP19). The interaction between PDI5 and two of these (RD21 and CP43) was confirmed by coimmunoprecipitation, and an in vitro Cys protease assay showed that PDI5 inhibited protease activity of RD21. Furthermore, colabeling immunoelectron microscopy demonstrated that PDI5 and RD21 associated with each other in endothelial cells (see figure ) and traveled together through the endomembrane system before the onset of PCD. Siliques of a pdi5 knockout mutant produced fewer seeds than those of the wild type and had reduced embryo viability, suggesting that PDI5 is required for normal seed development.
Activation of Cys proteases is an important trigger of PCD in animal cells (reviewed in Rupinder et al., 2007  ), and Cys proteases have been detected in plant cells undergoing PCD (Minami and Fukuda, 1995). The authors propose that PDI5 chaperones Cys proteases to the protein storage vacuoles of endothelial cells and blocks their destructive protease activity, by means of a thioredoxin-based mechanism, until the onset of PCD. Thus, PDI5 may be involved in regulating the timing of PCD. Footnotes www.plantcell.org/cgi/doi/10.1105/tpc.108.200810 REFERENCES
Greenberg, J.T. (1996). Programmed cell death: A way of life for plants. Proc. Natl. Acad. Sci. USA 93: 12094–12097. Minami, A., and Fukuda, H. (1995). Transient and specific expression of a cysteine endopeptidase associated with autolysis during differentiation of Zinnia mesophyll cells into tracheary elements. Plant Cell Physiol. 108: 489–493. Ondzighi, C.A., Christopher, D.A., Cho, E.J., Chang, S.-C., and Staehlin, L.A. (2008). Arabidopsis protein disulfide isomerase-5 inhibits cysteine proteases during trafficking to vacuoles before programmed cell death of the endothelium in developing seeds. Plant Cell 20: 2205–2220. Rupinder, S.K., Gurpreet, A.K., and Manjeet, S. (2007). Cell suicide and caspases. Vascul. Pharmacol. 46: 383–393.[CrossRef][ISI][Medline] Schmid, M., Simpson, D.J., Sarioglu, H., Lottspeich, F., and Gietl, C. (2001). The ricinosomes of senescing plant tissue bud from the endoplasmic reticulum. Proc. Natl. Acad. Sci. USA 98: 5353–5358. Related articles in Plant Cell:
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25% smaller than the other members and yet contains both of the characteristic thioredoxin catalytic domains. Using immunofluorescence microscopy, the scientists found that PDI5 accumulated in the endothelium (the layer of cells that surrounds the endosperm and undergoes PCD during embryogenesis). PDI5 was abundant in the endothelium of the pre-embryo stage ovule and declined as the embryo developed and the endothelium underwent PCD. Immunoelectron microscopy revealed that the bulk of PDI5 had accumulated in protein storage vacuoles by the heart stage of embryonic development. 