Exploring the effect of mechanical anisotropy of protein structures in the unfoldase mechanism of AAA+ molecular machines
Essential cellular processes of microtubule disassembly and protein degradation, which span lengths from tens of m to nm, are mediated by specialized molecular machines with similar hexameric structure and function. Our molecular simulations at atomistic and coarse-grained scales show that both the microtubule severing protein spastin and the caseinolytic protease ClpY, accomplish spectacular unfolding of their diverse substrates, a microtubule lattice and dihydrofolate reductase (DHFR), by taking advantage of mechanical anisotropy in these proteins. By considering wild-type and variants of DHFR, we found that optimal ClpY-mediated action probes favorable orientations of the substrate relative to the machine. Unfolding of wild-type DHFR involves strong mechanical interfaces near each terminal and occurs along branched pathways, whereas unfolding of DHFR variants involves softer mechanical interfaces and occurs through single pathways, but translocation hindrance can arise from internal mechanical resistance. For spastin, optimum severing action initiated by pulling on a tubulin subunit is achieved through the orientation of the machine versus the substrate (microtubule lattice). Moreover, changes in the strength of the interactions between spastin and a microtubule filament, which can be driven by the tubulin code, lead to drastically different outcomes for the integrity of the hexameric structure of the machine.