Fig. 1. Revealing the structural mechanics of in situ-assembled lamin filaments.

a A schematic illustration of the experimental set-up. Isolated nuclei from X. laevis oocytes were attached onto a poly-l-lysine-coated glass dish or carbon-coated electron microscopy grid. Next, the nuclei were manually opened by a sharp glass needle, and chromatin removed to expose the nucleoplasmic face. Zoom showing the cantilever tip pushing onto a lamin filament. For cryo-ET, an entire nucleus was collapsed on a grid by puncturing and removing the chromatin. b AFM imaging (nominal tip radius ≈10 nm) with a force of 0.5 nN. A representative image from the nucleoplasmic side showing areas of near-orthogonal arranged lamin filaments interspersed with NPCs. Randomly arranged filaments were also observed (Supplementary Fig. 2). Mechanical measurements on lamin filaments were performed on both meshwork types (N = 51, N is the number of independent experiments). Color scale denotes the topograph height. Scale bar, 100 nm. c Surface-rendered view of a cryo-electron tomogram (n = 13 tomograms) acquired on a spread NE of the X. laevis oocyte. Nuclear lamins formed a 3D meshwork of filaments (yellow) connected to NPCs (red). Field of view, 700 nm × 700 nm. d A typical FE signal showing the nonlinear behavior of a lamin filament in the meshwork. e In silico, a single lamin filament in the meshwork when subjected to mechanical push showed a comparable FE profile; a low-force regime and a steep rise were identified indicating strain-induced stiffening leading to failure of the lamin filament. The different regions were assigned to the molecular changes in the lamin α-helical coiled coils. The yield point denotes the point of plastic or permanent deformation, i.e., irreversible structural change. The similarity between the FE curves obtained in vitro and in silico suggests that single lamin filaments were probed.