Structure and function of BcpE2, the most promiscuous GH3-family beta-glucosidase for scavenging glucose from heterosides

Cellulose being the most abundant polysaccharide on earth, beta-glucosidases hydrolyzing cello-oligosaccharides are key enzymes to fuel glycolysis in microorganisms developing on plant material. In Streptomyces scabiei, the causative agent of common scab in root and tuber crops, a genetic compensation phenomenon safeguards the loss of the gene encoding the cello-oligosaccharide hydrolase BglC by awakening the expression of alternative beta-glucosidases. Here we reveal that the BglC compensating enzyme BcpE2 is the GH3-family beta-glucosidase that displays the highest reported substrate promiscuity able to release the glucose moiety of all tested types of plant-derived heterosides (aryl {beta}-glucosides, monolignol glucosides, cyanogenic glucosides, anthocyanosides, and coumarin heterosides). BcpE2 structure analysis highlighted a large cavity in the PA14 domain that covers the active site, and the high flexibility of this domain would allow proper adjustment of this cavity for disparate heterosides. The exceptional substrate promiscuity of BcpE2 provides microorganisms a versatile tool for scavenging glucose from plant-derived nutrients that widely vary in size and structure. Importantly, scopolin is the only substrate commonly hydrolyzed by both BglC and BcpE2 thereby generating the potent virulence inhibitor scopoletin. Next to fueling glycolysis, both enzymes thus also interfere with the plant defense mechanisms to fine-tune the strength of virulence.