Structural Characterization of the 16-kDa Allergen, RA17, in Rice Seeds. Prediction of the Secondary Structure and Identification of Intramolecular Disulfide Bridges

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  The 16-kDa rice allergen, RA17, belonging to the α-amylase/trypsin inhibitor family was isolated from rice seed and structurally characterized by identifying cystine-containing peptides and predicting the secondary structure and hydrophobic regions. Eight peptides, which constitute three sets of cystine-containing peptides, were purified by HPLC from a thermolytic digest of RA17 and identified by their amino acid sequence and composition, indicating five intramolecular disulfide bridges: Cys³⁴-Cys⁹⁴, Cys²⁶-(Cys⁵⁰ or Cys⁵¹)-Cys¹¹⁰ and Cys¹²-(Cys⁶² or Cys⁶⁴)-Cys¹²². Analyses of the CD spectrum and the Chou-Fasman prediction suggested that RA17 had some helical- and sheet-structure regions. Based on these experimental and predicted data, RA17 is proposed to be a globular molecule with a small hydrophobic core having folding restricted by five intramolecular disulfide bridges.