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Current structural understanding of kinases is largely based on x-ray crystallographic studies, whereas very little data exist on the conformations and dynamics that kinases adopt in the solution state. ABL kinase is an important drug target in the treatment of chronic myelogenous leukemia. Here, we present the first characterization of ABL kinase in complex with three clinical inhibitors (imatinib, nilotinib, and dasatinib) by modern solution NMR techniques. Structural and dynamical results were derived from complete backbone resonance assignments, experimental residual dipolar couplings, and (15)N relaxation data. Residual dipolar coupling data on the imatinib and nilotinib complexes show that the activation loop adopts the inactive conformation, whereas the dasatinib complex preserves the active conformation, which does not support contrary predictions based upon molecular modeling. Nanosecond as well as microsecond dynamics can be detected for certain residues in the activation loop in the inactive and active conformation complexes.

The crystal structure of the ligand binding domain (LBD) of the estrogen-related receptor α (ERRα, NR3B1) complexed with a coactivator peptide from peroxisome proliferator-activated receptor coactivator-1α (PGC-1α) reveals a transcriptionally active conformation in the absence of a ligand. This is the first x-ray structure of ERRα LBD, solved to a resolution of 2.5 A, and the first structure of a PGC-1α complex. The putative ligand binding pocket (LBP) of ERRα is almost completely occupied by side chains, in particular with the bulky side chain of Phe328 (corresponding to Ala272 in ERRγ and Ala350 in estrogen receptor α). Therefore, a ligand of a size equivalent to more than ∼4 carbon atoms could only bind in the LBP, if ERRα would undergo a major conformational change (in particular the ligand would displace H12 from its agonist position). The x-ray structure thus provides strong evidence for ligand-independent transcriptional activation by ERRα. The interactions of PGC-1α with ERRα also reveal for the first time the atomic details of how a coactivator peptide containing an inverted LXXLL motif (namely a LLXYL motif) binds to a LBD. In addition, we show that a PGC-1α peptide containing this nuclear box motif from the L3 site binds ERRα LBD with a higher affinity than a peptide containing a steroid receptor coactivator-1 motif and that the affinity is further enhanced when all three leucine-rich regions of PGC-1α are present.