OTU takes the chains OUT.

ERα LBD structures may explain instability in the absence of ligand or with specific ligands 1,2,7,9. Nettles et al. 5 address this instability through use of a specific point mutation, Y537S, which stabilizes H12 in an agonist con-formation and is shown to overcome misfold-ing of the apo-ERα LBD. One might predict that stabilizing H12 on top of the ligand binding cavity with the Y537S mutation would prevent efficient access of ligands to the binding pocket. Unexpectedly, analysis of apo Y537S ERα crystals suggests that ligand entry can take place through a previously undescribed channel created by a switch in the position of His524, which normally closes this channel in the presence of ligand. Accordingly, mutant Y537S was amenable to the rapid generation of more than ten structures via cocrystallization and soaking strategies. Since this channel appears conserved in all steroid receptors, use of mutations stabilizing H12 in the agonist conformation may be generally applicable to structural studies of this class of nuclear receptors. Importantly, the mutation introduced did not affect the interaction of the receptor with ligands, and structures obtained with the mutant receptor could be superimposed to those previously generated with wild-type ERα. However, the restriction imposed on the positioning of H12 may limit the application of this strategy with antagonists having bulky side chains. Instead, a different mutation (L536S) was designed to stabilize H12 in the antagonist conformation seen in the presence of tamoxi-fen and raloxifene (Fig. 1) 1,2 , and this mutation was shown to be compatible with binding of raloxifene in a cocrystallization approach. It will be of interest to determine whether the L536S mutation allows apo receptor crystal-lization and soaking with a complementary range of ligands. Use of different stabilized structures together with monitoring of the antagonist/agonist properties of ligands in mutant versus wild-type receptors may also provide information on H12 dynamics in the presence of these ligands. However, whether some ligands such as full antiestrogens are compatible with H12-stabilizing mutations remains at present an open question, as H12 was found to be displaced and unstructured with this type of ligand 7. Nettles et al. 5 illustrate the utility of their approach by solving the structure of Y537S ERα with several NFκB-selective ligands (Fig. 1), revealing the basis for their destabilizing effect on the ERα LBD conformation. These ligands retain the capacity of natural estrogens to suppress NFκB activity in an ERα-dependent manner, but they …