Catalytic subunit exchanges in the cellulosomes produced by Ruminiclostridiumcellulolyticum suggest unexpected dynamics and adaptability of their enzymatic composition.

Abstract

Cellulosomes are complex nanomachines produced by cellulolytic anaerobic bacteria such as Ruminiclostridiumcellulolyticum (formerly known as Clostridiumcellulolyticum). Cellulosomes are composed of a scaffoldin protein displaying several cohesin modules on which enzymatic components can bind to through their dockerin module. Although cellulosomes have been studied for decades, very little is known about the dynamics of complex assembly. We have investigated the ability of some dockerin-bearing enzymes to chase the catalytic subunits already bound onto a miniscaffoldin displaying a single cohesin. The stability of the preassembled enzyme-scaffoldin complex appears to depend on the nature of the dockerin, and we have identified a key position in the dockerin sequence that is involved in the stability of the complex with the cohesin. Depending on the residue occupying this position, the dockerin can establish with the cohesin partner either a nearly irreversible or a reversible interaction, independently of the catalytic domain associated with the dockerin. Site-directed mutagenesis of this residue can convert a dockerin able to form a highly stable complex with the miniscaffoldin into a reversible complex forming one and vice versa. We also show that refunctionalization can occur with natural purified cellulosomes. Altogether, our results shed light on the dynamics of cellulosomes, especially their capacity to be remodeled even after their assembly is 'achieved', suggesting an unforeseen adaptability of their enzymatic composition over time.