Hydrolysis of Storage Proteins in Barley Endosperms: Role of Gibberellic Acid

Abstract

<p>The storage proteins in barley endosperms, hordeins, are insoluble in aqueous buffers. During early seedling growth, they are hydrolyzed to small peptides and amino acids. The overall hydrolysis of endosperm reserves is dependent, on enzymes, such as alpha-amylases, carboxypeptidases, and endoproteases, which are induced in response to gibberellic acid. Soluble products released from the hydrolysis of hordeins were analyzed to determine: a) the mode of hydrolysis of these proteins; b) the role of gibberellic acid in the initial hydrolysis of hordein proteins; and finally c) whether the initial events in the hydrolysis are mediated by a specific protease. Soluble proteins were prepared from dry endosperms (controls) and embryo-less endosperm pieces incubated for 24 and 72 hours in a buffered medium in the presence and absence of gibberellic acid. The hordein-related polypeptides in the soluble fraction were identified by using IgG's prepared against urea-denatured hordeins. In the control samples, several bands were detected on western immunoblots. Additional bands in the size range of 25-30 kilodaltons and 40 kilodaltons appeared in the absence of gibberellic acid. In the presence of gibberellic acid, however, fragments in the size range of less than 15 kilodaltons were more dominant. Furthermore, the levels of small peptides (less than 30-35 amino acid residues in length) and free amino acids increased when samples were incubated for longer times in the presence of gibberellic acid. The results are consistent with the idea that a gibberellic acid-independent endoprotease(s) is involved in the initial stages of hordein hydrolysis. Further hydrolysis is, however, dependent on the presence of proteases induced by gibberellic acid. The results are discussed in relation to both the mode of hydrolysis and the specificity of initial events of hordein hydrolysis.</p>