Adakkattil, R.; Mandal, P.; Savich, Y.; Ruffine, V.; Jordan, M. A.; Pinholt, H. D.; Fischer-Friedrich, E.; Julicher, F.; Grill, S. W.; Von Appen, A.

The nuclear envelope protects the genome from mechanical stress during processes such as migration, division, and compression, but how it buffers forces at the scale of DNA remains unclear. Here, we utilize optical tweezers to show that a multivalent protein DNA co-condensate containing the nuclear envelope protein LEM2 and the DNA-binding protein BAF shield DNA beyond its melting point at 65 pN. Under load, their collective assembly induces an unconventional DNA stiffening effect that provides mechanical reinforcement, dependent on the intrinsically disordered region (IDR) of LEM2. Within cells, these components form an elastic surface hydrogel at the nuclear periphery, visible as a continuous surface by cryo-electron tomography. Disruption of this surface hydrogel increases DNA damage and micronuclei formation during nuclear deformation. Together, this work expands the functional repertoire of condensates, revealing a load-responsive nuclear surface hydrogel at the mesoscale that mitigates mechanical stress.