Unprocessed barley aleurone endo-β-1,4-xylanase X-I is an active enzyme

Endo-β-1,4-xylanase X-I is a major hydrolase produced by the aleurone tissue of germinating barley grain. It was previously reported that this cytosolic enzyme is synthesized as an inactive precursor which is proteolytically processed to active forms upon its programmed cell death dependent release. We here demonstrate, however, that the precursor form of X-I is an active enzyme. Purified recombinant precursor X-I was characterised with respect to its molecular weight, iso-electric point and temperature and pH activity and stability. Analysis of the hydrolysis products showed that it is an endo-acting enzyme which has the striking ability to release xylose from both polymeric xylan as well as from small xylo-oligosaccharides. The implications of these findings in relation to the putative role of the N-terminal propeptide as a carbohydrate binding module and the possible consequences for the way X-I fulfils its role in the germination process, are discussed.