The DBD and hydrophobic NES use a common or overlapping binding site on CRT. (A) Binding assay with GST-DBD or GST (2 μg each) immobilized on glutathione beads and CRT (500 ng), RanGTP (2 μg), and WT or MUT PKI (2 μg) added in solution. The bound fractions were analyzed by immunoblotting for CRT. Including WT PKI in the reaction reduced the level of CRT bound to the DBD, indicating that these proteins bind to similar sites on CRT. This competition was not observed when RanGTP was omitted from the assay (data not shown), consistent with Ran acting as a cofactor for NES binding but not for DBD binding. (B) Nuclear export of GR-GFP was assayed in permeabilized cells using CRT (2.0 μM) in the presence of buffer, excess DBD (4.5 μM), or PKI (12 μM WT or MUT). (C) Competitive binding interactions between NES, DBD, and CRT measured in a biosensor assay. NES peptide was immobilized on the cuvette surface through a biotin-neutravidin linkage and was used to measure the Ran-dependent binding of CRT in the absence and presence of DBD in the solution. The proteins used in the assay were CRT (1.1 μM), Crm1 (1.1 μM), RanQ69L (1.9 μM), and WT and MUT DBD from GR (9.1μM each). CRT binds efficiently to NES peptide in the presence of Ran (green tracing), and this can be competed with the WT DBD (light blue tracing) but not with the transport-defective MUT DBD that contains the FF-to-AA mutations (dark blue tracing). Crm1 binding to the NES is unaffected by the presence of excess DBD (fuchsia tracing). (D) The DBD competes with NES in the CRT-dependent export pathway. The cell line expressing Rev-GFP was used to assay export in the presence of CRT (1.1 μM), WT PKI (12 μM), and WT DBD (9.1 μM) as indicated.