Fig 1: A fraction of Y complexes colocalizes with the transmembrane nucleoporin POM121 in the mitotic ER ER‐associated Y‐complexes are in close proximity to ER tubule and sheet markers. Confocal images of HeLa cells expressing GFP‐Nup107 that were arrested at the metaphase–anaphase transition. Cells were either fixed (untreated) or first semipermeabilized and then fixed, as described in Fig 3. ER sheets and tubules were visualized by immunostaining with antibodies directed to Climp63 or RTN4. DNA was stained with Hoechst, not shown in the merged images. Representative images of three biological replicates. Scale bar, 10 μm and inset scale bar, 1 μm.HeLa cells expressing GFP‐Nup107 were treated as in (A) and immunostained with an antibody targeting POM121. Merged channels exclude Hoechst staining. Representative images of three biological replicates. Scale bar, 10 μm and inset scale bar, 1 μm.