Post-translational N- and C-terminal processing in the development of wheat leaf beta-amylase polymorphism from an hitherto undetected primary gene product
2002
Zemanova, L. ((Universitaet Bayreuth (Allemagne). Lehrstuhl Pflanzenphysiologie)) | Peter-Katalinic, J. | Koenig, S. | Wagner, G. | Ziegler, P.
The beta-amylase (EC 3.2.1.2) of wheat (Triticum aestivum L.) leaves comprises at least five distinguishable isoforms (betaI-betaV) apparently corresponding to a beta-amylase-encoding cDNA obtained from leaf mRNA (cDNA1). This beta-amylase has now been purified by immunoaffinity chromatography on antibodies directed against the heterologous expression product of cDNA1. Upon resolution into component forms by anion exchange chromatography, a new beta-amylase form larger than any of betaI-betaV was discovered (betaN). The electrophoretic patterns and amino acid sequences of peptides yielded by tryptophan-specific digestion confirmed that the leaf beta-amylase isoforms all correspond to cDNA1. C-terminal peptides of the isoforms showed the same size progression as did the intact proteins (betaN betaV betaIV betaIII betaII betaI). According to mass spectrometry, betaV, betaIII/betaII and betaI represent truncations of 8, 12 and not more than 14 amino acids, respectively, of the C-terminus of betaN. All of the beta-amylase forms begin N-terminally with the (acetylated) Ala2 of the amino acid sequence predicted by cDNA1, although some of each of betaI-betaIII also commence with Met5. A leaf beta-amylase-encoding gene corresponding to cDNA1 is thus translated under removal of the initiator methionine and acetylation of the newly formed N-terminus to produce betaN. betaI-betaV are then formed from betaN by the removal of up to 14 amino acids from the C-terminus, concomitant to the removal of three further N-terminal amino acids during the formation of some of betaI-betaIII
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