Identification of Exp4 as a major interaction partner of Sox9.

(A) Silver staining of Sox9 binding proteins separated by NuPAGE. Nuclear extracts prepared from HeLa cells (HeLa NE) were incubated with (lanes 3, 4) or without FLAG-tagged Sox9 (FLAG-Sox9, lanes 1, 2). After recovery with anti-FLAG M2 antibody-conjugated agarose, the proteins were subjected to NuPAGE. The closed arrowhead indicates FLAG-Sox9, and the open arrowhead indicates the protein that was specifically recovered by FLAG-Sox9 (lane 4). (B) Nuclear extracts from U2OS cells were subjected to immunoprecipitation with anti-Sox9 antibody, and the precipitates were subjected to Western blotting analysis using anti-Exp4 antibody (right lane). Normal rabbit IgG was used as a control (middle lane). 1% of the nuclear extract was applied as a control (left lane). (C) The schematic depicts the truncated forms of Sox9 fused with GST (dark gray boxes). The numbers indicate the amino acid residues. The HMG box domain is shown as a light gray box (103–181 a.a.). (D) The upper panel shows Western blotting analysis of the protein samples co-precipitated with GST-fused truncated forms of Sox9 using an anti-Exp4 antibody. 5% of the nuclear extract was applied as a control (left lane). Numbers represent the corresponding GST-fused truncated Sox9 constructs shown in C. The lower panel shows CBB staining of NuPAGE for the GST fusion proteins used in this experiment. Numbers on the right represent the molecular weights of the marker proteins.