Synthesis, processing and export of cytoplasmic endo-beta-1,4-xylanase from barley aleurone during germination

We have identified the major endo-beta -1,4-xylanase (XYN-1) in the aleuron e of germinating barley grain, and show that it is expressed as a precursor of M-r 61500 with both N- and C-terminal propeptides. XYN-1 is synthesized as an inactive enzyme in the cytoplasm, and only becomes active at a late stage of germination when the aleurone ceases to secrete hydrolases. A seri es of processing steps, mediated in part by aleurone cysteine endoproteases , yields a mature active enzyme of M-r 34 000. Processing and extracellular release of the mature enzyme coincide with the programmed cell death (PCD) -regulated disintegration of aleurone cells. We discuss the significance of delayed aleurone cell-wall degradation by endoxylanases in relation to the secretory capacity of the aleurone, and propose a novel role for aleurone PCD in facilitating the export of hydrolases.