BIOCHEMICAL AND MOLECULAR CHARACTERIZATION OF A BARLEY SEED BETA-GLUCOSIDASE

A 60-kDa beta-glucosidase (BGQ60) was purified and characterized from seeds of barley (Hordeum vulgare L.) BGQ60 catalytic activity was rest ricted to the cleavage of short-chain oligosaccharides composed of (1- 2)-, (1-3)-, and/or (1-4)-beta-linked glucose or mannose units. These oligosaccharides are the primary products of endosperm cell wall polys accharide hydrolysis by other enzymes. In keeping with this, complete hydrolysis of the major polysaccharide of barley starchy endosperm cel l wall, (1.3, 1-4)-beta-glucan, to free glucose was shown to require t he concerted action of endo-(1-3, 1-4)-beta-glucanase and BGQ60. The c omplete amino acid sequence of BGQ60 was determined by protein sequenc ing combined with the deduced sequence of the corresponding cDNA and g enomic clones. The BGQ60 primary structure exhibits extensive homology to members of glycosyl hydrolase family 1 (EC 3.2.1.21). Southern and Northern blot analysis with the cDNA as probe indicated that BGQ60 is encoded by a single gene, and that BGQ60 mRNA only accumulates in the starchy endosperm tissue of late developing seeds. The bgq60 structur al gene of approximately kilobases contains an open reading frame enco ding 485 amino acids interrupted by 9 introns. The complete nucleotide sequence of the bgq60 structural gene represents the first characteri zed plant gene encoding a beta-glucosidase. The barley BGQ60 is a nove l plant beta-glucosidase with a hitherto undescribed specific enzymati c activity. The possible biological functions of BGQ60 during barley s eed development and germination are discussed.