title: Maize tapetum xylanase is synthesized as a precursor, processed and activated by a serine protease, and deposited on the pollen.
abstract: Pollen coat contains ingredients that interact with the stigma surface during sexual reproduction. In maize (Zea mays L.) pollen coat, the predominant protein is a 35-kDa endoxylanase, whose mRNA is located in the tapetum cells enclosing the maturing pollen in the anthers. This 2.0-kb mRNA was found to have an open reading frame of 1,635 nucleotides encoding a 60-kDa pre-xylanase. In developing anthers, the pre-xylanase protein appeared prior to the 35-kDa xylanase protein and enzyme activity and then peaked and declined, whereas the 35-kDa xylanase protein and activity continued to increase until anther maturation. An acid protease in the anther extract converted the inactive pre-xylanase to the active 35-kDa xylanase in vitro. The protease activity was inhibited by inhibitors of serine proteases but unaffected by inhibitors of cysteine, aspartic, or metallic proteases. Sequence analysis revealed that the 60-kDa pre-xylanase was converted to the 35-kDa xylanase with the removal of 198 and 48 residues from the N and C termini, respectively. During in vitro and in vivo conversions, no intermediates of 60-35 kDa were observed, and the 35-kDa xylanase was highly stable. The pre-xylanase was localized in the tapetum-containing anther wall, whereas the 35-kDa xylanase was found in the pollen coat. The significance of having a large non-active pre-xylanase and the mode of transfer of the xylanase to the pollen coat are discussed. A gene encoding the barley (Hordeum vulgare L.) tapetum xylanase was cloned; this gene and the gene encoding the seed aleurone-layer xylanase had strict tissue-specific expressions.
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D018744:DNA, Plant; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000096:biosynthesis* / Q000378:metabolism; D001467:Hordeum / Q000235:genetics; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D011058:Pollen / Q000201:enzymology*; D012333:RNA, Messenger / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D043325:Xylan Endo-1,3-beta-Xylosidase; D014995:Xylosidases / Q000037:antagonists & inhibitors / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D003313:Zea mays / Q000201:enzymology*
chemical_list: D018744:DNA, Plant; D004792:Enzyme Precursors; D012333:RNA, Messenger; D014995:Xylosidases; D043325:Xylan Endo-1,3-beta-Xylosidase