Crystal Structures of the Human Doublecortin C- and N-terminal Domains in Complex with Specific Antibodies.

Doublecortin is a microtubule-associated protein produced during neurogenesis. The protein stabilizes microtubules and stimulates their polymerization, which allows migration of immature neurons to their designated location in the brain. Mutations in the gene that impair doublecortin function and cause severe brain formation disorders are located on a tandem repeat of two doublecortin domains. The molecular mechanism of action of doublecortin is only incompletely understood. Anti-doublecortin antibodies, such as the rabbit polyclonal Abcam 18732, are widely used as neurogenesis markers. Here, we report the generation and characterization of antibodies that bind to single doublecortin domains. The antibodies were used as tools to obtain structures of both domains. Four independent crystal structures of the N-terminal domain reveal several distinct open and closed conformations of the peptide linking N- and C-terminal domains, which can be related to doublecortin function. An NMR assignment and a crystal structure in complex with a camelid antibody fragment show that the doublecortin C-terminal domain adopts the same well defined ubiquitin-like fold as the N-terminal domain, despite its reported aggregation and molten globule-like properties. The antibodies' unique domain specificity also renders them ideal research tools to better understand the role of individual domains in doublecortin function. A single chain camelid antibody fragment specific for the C-terminal doublecortin domain affected microtubule binding, whereas a monoclonal mouse antibody specific for the N-terminal domain did not. Together with steric considerations, this suggests that the microtubule-interacting doublecortin domain observed in cryo-electron micrographs is the C-terminal domain rather than the N-terminal one.

Cholesteryl ester transfer between lipoproteins does not require a ternary tunnel complex with CETP.

The cholesteryl ester transfer protein (CETP) enables the transfer of cholesteryl ester (CE) from high-density lipoproteins (HDL) to low-density lipoproteins (LDL) in the plasma compartment. CETP inhibition raises plasma levels of HDL cholesterol; a ternary tunnel complex with CETP bridging HDL and LDL was suggested as a mechanism. Here, we test whether the inhibition of CETP tunnel complex formation is a promising approach to suppress CE transfer from HDL to LDL, for potential treatment of cardio-vascular disease (CVD). Three monoclonal antibodies against different epitopes of CETP are assayed for their potential to interfere with CE transfer between HDL and/or LDL. Surprisingly, antibodies that target the tips of the elongated CETP molecule, interaction sites sterically required to form the suggested transfer complexes, do not interfere with CETP activity, but an antibody binding to the central region does. We show that CETP interacts with HDL, but not with LDL. Our findings demonstrate that a ternary tunnel complex is not the mechanistic prerequisite to transfer CE among lipoproteins.

Insight into steroid scaffold formation from the structure of human oxidosqualene cyclase.

In higher organisms the formation of the steroid scaffold is catalysed exclusively by the membrane-bound oxidosqualene cyclase (OSC; lanosterol synthase). In a highly selective cyclization reaction OSC forms lanosterol with seven chiral centres starting from the linear substrate 2,3-oxidosqualene. Valuable data on the mechanism of the complex cyclization cascade have been collected during the past 50 years using suicide inhibitors, mutagenesis studies and homology modelling. Nevertheless it is still not fully understood how the enzyme catalyses the reaction. Because of the decisive role of OSC in cholesterol biosynthesis it represents a target for the discovery of novel anticholesteraemic drugs that could complement the widely used statins. Here we present two crystal structures of the human membrane protein OSC: the target protein with an inhibitor that showed cholesterol lowering in vivo opens the way for the structure-based design of new OSC inhibitors. The complex with the reaction product lanosterol gives a clear picture of the way in which the enzyme achieves product specificity in this highly exothermic cyclization reaction.

Molecular switch in the glucocorticoid receptor: active and passive antagonist conformations.

Mifepristone is known to induce mixed passive antagonist, active antagonist, and agonist effects via the glucocorticoid receptor (GR) pathway. Part of the antagonist effects of mifepristone are due to the repression of gene transcription mediated by the nuclear receptor corepressor (NCoR). Here, we report the crystal structure of a ternary complex of the GR ligand binding domain (GR-LBD) with mifepristone and a receptor-interacting motif of NCoR. The structures of three different conformations of the GR-LBD mifepristone complex show in the oxosteroid hormone receptor family how helix 12 modulates LBD corepressor and coactivator binding. Differences in NCoR binding and in helix 12 conformation reveal how the 11beta substituent in mifepristone triggers the helix 12 molecular switch to reshape the coactivator site into the corepressor site. Two observed conformations exemplify the active antagonist state of GR with NCoR bound. In another conformation, helix 12 completely blocks the coregulator binding site and explains the passive antagonistic effect of mifepristone on GR.

Enhancing the stability and solubility of the glucocorticoid receptor ligand-binding domain by high-throughput library screening.

The human glucocorticoid receptor ligand-binding domain (hGR-LBD) is an important drug target for the treatment of various diseases. However, the low intrinsic stability and solubility of hGR-LBD have rendered its purification and biophysical characterization difficult. In order to overcome these problems, we have stabilized hGR-LBD by a combination of random mutagenesis and high-throughput screening using fluorescence-activated cell sorting (FACS) with enhanced green fluorescent protein (eGFP) as folding reporter. Two plasmid-encoded gene libraries of hGR-LBD fused to the egfp gene were expressed in Escherichia coli, followed by eight rounds of FACS screening, in each of which 10(8) cells were analyzed. The hgr-lbd mutants isolated by this approach contained numerous amino acid exchanges, and four beneficial ones (A605V, V702A, E705G, and M752T) were followed up in detail. Their characterization showed that the fluorescence of hGR-LBD-eGFP fusions is correlated linearly with the stability and solubility of hGR-LBD in the absence of eGFP. When combined, the four exchanges increased the thermal stability of hGR-LBD by more than 8 °C and enhanced its purification yield after expression in E. coli by about 26-fold. The introduction of three beneficial exchanges into the homologous ligand-binding domain of mouse enabled its X-ray structure determination at high resolution, which showed how the exchanges stabilize the protein and revealed atomic details that will guide future drug design. Our results demonstrate that large eGFP fusion libraries can be screened by FACS with extreme sensitivity and efficiency, yielding stabilized eukaryotic proteins suitable for biophysical characterization and structure determination.

The monotopic membrane protein human oxidosqualene cyclase is active as monomer.

The monotopic integral membrane protein 2,3-oxidosqualene cyclase (OSC) catalyzes the formation of lanosterol the first sterol precursor of cholesterol in mammals. Therefore, it is an important target for the development of new hypocholesterolemic drugs. Here, we report the overexpression and purification of functional human OSC (hOSC) in Pichia pastoris. The obtained IC(50) for the reference inhibitor Ro 48-8071 is nearly identical for the recombinant hOSC compared to OSC from human liver microsomes. The correlation of analytical ultracentrifugation data and activity measurements showed the highest enzymatic activity for the monomeric hOSC indicating that this would be the natural form. Furthermore, these data helped us to identify the detergent for a successful crystallization of the protein. The availability of this active recombinant human membrane protein is a very important step on the way to a more detailed functional and structural characterization of OSCs.