Hendrix, J. — Page 2 | bioRxiv Science

bioRxiv2019

Nucleotide-mediated allosteric regulation of bifunctional Rel enzymes

Bifunctional Rel stringent factors, the most broadly distributed class of RSHs, are ribosome-associated enzymes that transfer a pyrophosphate group from ATP onto the 3' of GTP or GDP to synthesize (p)ppGpp and also catalyse the 3' pyrophosphate hydrolysis of the alarmone to degrade it. The precise regulation of these enzymes seems to be a complex allosteric mechanism, and despite decades of research, it is unclear how the two opposing activities of Rel are controlled at the molecular level. Here we show that a stretch/recoil guanosine-switch mechanism controls the catalytic cycle of T. thermophilus Rel (RelTf). The binding of GDP/ATP stretches apart the NTD catalytic domains of RelTf (RelTtNTD) activating the synthetase domain and allosterically blocking the hydrolase active site. Conversely, binding of ppGpp unlocks the hydrolase domain and triggers recoil of both NTDs, which partially buries the synthetase active site and precludes the binding of synthesis precursors. This allosteric mechanism acts as an activity switch preventing futile cycles of alarmone synthesis and degradation.

molecular biology↗

bioRxiv2018

Dynamic interactions of type I cohesin modules fine-tune the structure of the cellulosome of Clostridium thermocellum

Efficient degradation of plant cell walls by selected anaerobic bacteria is performed by large extracellular multienzyme complexes termed cellulosomes. The spatial arrangement within the cellulosome is organized by a protein called scaffoldin, which recruits the cellulolytic subunits through interactions between cohesin modules on the scaffoldin and dockerin modules on the enzymes. Although many structural studies of the individual components of cellulosomal scaffoldins have been performed, the role of interactions between individual cohesin modules and the flexible linker regions between them are still not entirely understood. Here, we report single-molecule measurements using Forster resonance energy transfer to study the conformational dynamics of a bimodular tandem cohesin segment of the scaffoldin protein CipA of Clostridium thermocellum. Our data reveal the existence of compacted structures in solution that persist on the timescale of milliseconds. The compacted conformation is found to be in dynamic equilibrium with an extended state that shows distance fluctuations on the microsecond timescale. Shortening of the inter-cohesin linker does not significantly alter the structural dynamics. Upon addition of dockerin-containing enzymes, an extension of the flexible state is observed but the cohesin-cohesin interactions persist. This suggests that the dockerin-binding interfaces are not involved in cohesin-cohesin interactions. The formation of cohesin-cohesin interactions is also observed in all-atom molecular dynamics simulations of the system. From the simulations, we identify possible inter-cohesin binding modes, none of which show obstruction of the cohesin-dockerin binding interfaces. Our results go beyond the view of scaffoldin as \"beads on a string\". We propose that both the flexibility and cohesin-cohesin interactions are important factors for the precise spatial arrangement of the enzymatic subunits in the cellulosome that leads to the high catalytic synergy in these assemblies. Hence, the flexibility of the linker region and cohesin-cohesin interactions should be considered when designing cellulosomes for industrial applications.

biophysics↗