Identification of IK binding proteins. A. A two-step pull-down procedure followed by SDS-PAGE was carried out as described in Materials and methods. The 3 squares indicate proteins in the areas that were subjected to analysis with mass spectrometry. B. HeLa cells were transfected with the GFP-IK expressing construct for 2 days. Cells were then fixed and stained with antibodies to GFP and coilin, a Cajal body component, and specific signals were examined by fluorescence microscopy.