Fig. 4. Asymmetric distribution of Sun1 and Nesprin-1 at the nuclear membrane. (A-C) Triton X-100 treated COS7 cells subjected to immunofluorescence with Sun1 and lamin A/C antibodies, indicate the nuclear localisation of Sun1 (arrows). Non-specific staining of antibody 281 was observed in the cytoplasm (arrowheads; see also Fig. 3C). (D-F) In digitonin-treated COS7 cells only the non-specific staining remains (arrowheads) suggesting a localisation of Sun1 at the inner nuclear membrane. The integrity of the nuclear membrane is documented by the absence of lamin A/C staining (E). (G-I) In Triton X-100-permeabilised fibroblasts Nesprin-1 antibodies strongly stain the nucleus. (J-L) Nesprin-1 staining at the NE persists after digitonin treatment suggesting the presence of Nesprin-1 at the outer nuclear membrane. Note the absence of lamin A/C staining (K). DAPI was used to counterstain the nucleus. Confocal images are shown. Bars, 5 µm.