Pre-catalytic activation pathway of the Smc5/6 complex
The Smc5/6 complex (Smc5/6) preserves genome stability through ATP-dependent DNA organization, but the structural details that prime the complex for productive DNA engagement remain unknown. Here, we combine cryo-electron microscopy and conformational heterogeneity analysis to define the pre-catalytic activation pathway of Smc5/6. We show that in the absence of ATP and DNA, Smc5/6 exists as a heterogeneous conformational ensemble composed predominantly of two conformations termed Bridge and Cross. In the presence of DNA alone, Smc5/6 adopts a Juxtaposed conformation that brings the ATPase heads into proximity, aligning them for nucleotide engagement. With both ATP and DNA associated, Smc5/6 establishes an Engaged architecture, within which DNA stabilizes its head-associated binding cavity through local remodeling of the complex. Conformational heterogeneity analyses suggest a continuous structural transition between the Cross and Juxtaposed states, while anisotropic network modeling identifies the Bridge-Cross-Juxtaposed-Engaged sequence as the mechanically favored pathway connecting the four experimentally observed conformations. Together, these findings support a pre-catalytic conformational pathway for Smc5/6 activation and provide a structural framework for understanding how conformational heterogeneity gives rise to an activation-competent motor state.