Youngchang Kim

Double chromodomains cooperate to recognize the methylated histone H3 tail.

Chromodomains are modules implicated in the recognition of lysine-methylated histone tails and nucleic acids. CHD (for chromo-ATPase/helicase-DNA-binding) proteins regulate ATP-dependent nucleosome assembly and mobilization through their conserved double chromodomains and SWI2/SNF2 helicase/ATPase domain. The Drosophila CHD1 localizes to the interbands and puffs of the polytene chromosomes, which are classic sites of transcriptional activity. Other CHD isoforms (CHD3/4 or Mi-2) are important for nucleosome remodelling in histone deacetylase complexes. Deletion of chromodomains impairs nucleosome binding and remodelling by CHD proteins. Here we describe the structure of the tandem arrangement of the human CHD1 chromodomains, and its interactions with histone tails. Unlike HP1 and Polycomb proteins that use single chromodomains to bind to their respective methylated histone H3 tails, the two chromodomains of CHD1 cooperate to interact with one methylated H3 tail. We show that the human CHD1 double chromodomains target the lysine 4-methylated histone H3 tail (H3K4me), a hallmark of active chromatin. Methylammonium recognition involves two aromatic residues, not the three-residue aromatic cage used by chromodomains of HP1 and Polycomb proteins. Furthermore, unique inserts within chromodomain 1 of CHD1 block the expected site of H3 tail binding seen in HP1 and Polycomb, instead directing H3 binding to a groove at the inter-chromodomain junction.

In situ proteolysis for protein crystallization and structure determination

We tested the general applicability of in situ proteolysis to form protein crystals suitable for structure determination by adding a protease (chymotrypsin or trypsin) digestion step to crystallization trials of 55 bacterial and 14 human proteins that had proven recalcitrant to our best efforts at crystallization or structure determination. This is a work in progress; so far we determined structures of 9 bacterial proteins and the human aminoimidazole ribonucleotide synthetase (AIRS) domain.

Structural basis for inhibition of the replication licensing factor Cdt1 by geminin

To maintain chromosome stability in eukaryotic cells, replication origins must be licensed by loading mini-chromosome maintenance (MCM2–7) complexes once and only once per cell cycle. This licensing control is achieved through the activities of geminin and cyclin-dependent kinases. Geminin binds tightly to Cdt1, an essential component of the replication licensing system, and prevents the inappropriate reinitiation of replication on an already fired origin. The inhibitory effect of geminin is thought to prevent the interaction between Cdt1 and the MCM helicase. Here we describe the crystal structure of the mouse geminin–Cdt1 complex using tGeminin (residues 79–157, truncated geminin) and tCdt1 (residues 172–368, truncated Cdt1). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdt1. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdt1 and, in particular, Pro 181, Ala 182, Tyr 183, Phe 186 and Leu 189. The crystal structure, in conjunction with our biochemical data, indicates that the N-terminal region of tGeminin might be required to anchor tCdt1, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdt1 through steric hindrance.

The active site of the SET domain is constructed on a knot.

The SET domain contains the catalytic center of lysine methyltransferases that target the N-terminal tails of histones and regulate chromatin function. Here we report the structure of the SET7/9 protein in the absence and presence of its cofactor product, S-adenosyl-L-homocysteine (AdoHcy). A knot within the SET domain helps form the methyltransferase active site, where AdoHcy binds and lysine methylation is likely to occur. A structure-guided comparison of sequences within the SET protein family suggests that the knot substructure and active site environment are conserved features of the SET domain.

The structural basis for substrate anchoring, active site selectivity, and product formation by P450 PikC from Streptomyces venezuelae

The pikromycin (Pik)/methymycin biosynthetic pathway of Streptomyces venezuelae represents a valuable system for dissecting the fundamental mechanisms of modular polyketide biosynthesis, aminodeoxysugar assembly, glycosyltransfer, and hydroxylation leading to the production of a series of macrolide antibiotics, including the natural ketolides narbomycin and pikromycin. In this study, we describe four x-ray crystal structures and allied functional studies for PikC, the remarkable P450 monooxygenase responsible for production of a number of related macrolide products from the Pik pathway. The results provide important new insights into the structural basis for the C10/C12 and C12/C14 hydroxylation patterns for the 12-(YC-17) and 14-membered ring (narbomycin) macrolides, respectively. This includes two different ligand-free structures in an asymmetric unit (resolution 2.1 Å) and two co-crystal structures with bound endogenous substrates YC-17 (resolution 2.35 Å)or narbomycin (resolution 1.7 Å). A central feature of the enzyme-substrate interaction involves anchoring of the desosamine residue in two alternative binding pockets based on a series of distinct amino acid residues that form a salt bridge and a hydrogen-bonding network with the deoxysugar C3′ dimethylamino group. Functional significance of the salt bridge was corroborated by site-directed mutagenesis that revealed a key role for Glu-94 in YC-17 binding and Glu-85 for narbomycin binding. Taken together, the x-ray structure analysis, site-directed mutagenesis, and corresponding product distribution studies reveal that PikC substrate tolerance and product diversity result from a combination of alternative anchoring modes rather than an induced fit mechanism.

Automation of protein purification for structural genomics.

AbstractA critical issue in structural genomics, and in structural biology in general, is the availability of high-quality samples. The additional challenge in structural genomics is the need to produce high numbers of proteins with low sequence similarities and poorly characterized or unknown properties. ‘Structural-biology-grade’ proteins must be generated in a quantity and quality suitable for structure determination experiments using X-ray crystallography or nuclear magnetic resonance (NMR). The choice of protein purification and handling procedures plays a critical role in obtaining high-quality protein samples. The purification procedure must yield a homogeneous protein and must be highly reproducible in order to supply milligram quantities of protein and/or its derivative containing marker atom(s). At the Midwest Center for Structural Genomics we have developed protocols for high-throughput protein purification. These protocols have been implemented on AKTA EXPLORER 3D and AKTA FPLC 3D workstations capable of performing multidimensional chromatography. The automated chromatography has been successfully applied to many soluble proteins of microbial origin. Various MCSG purification strategies, their implementation, and their success rates are discussed in this paper. abbreviations MCSG — Midwest Center for Structural Genomics; IMAC — immobilized metal affinity chromatography; TEV — tobacco etch virus; —β-ME —β-mercaptoethanol; DTT — dithiothreitol; EDTA — ethylenediaminetetraacetate; SDS-PAGE — polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate.

Multidomain integration in the structure of the HNF-4α nuclear receptor complex

The hepatocyte nuclear factor 4α (HNF-4α; also known as NR2A1) is a member of the nuclear receptor (NR) family of transcription factors, which have conserved DNA-binding domains and ligand-binding domains. HNF-4α is the most abundant DNA-binding protein in the liver, where some 40% of the actively transcribed genes have a HNF-4α response element. These regulated genes are largely involved in the hepatic gluconeogenic program and lipid metabolism. In the pancreas HNF-4α is also a master regulator, controlling an estimated 11% of islet genes. HNF-4α protein mutations are linked to maturity-onset diabetes of the young, type 1 (MODY1) and hyperinsulinaemic hypoglycaemia. Previous structural analyses of NRs, although productive in elucidating the structure of individual domains, have lagged behind in revealing the connectivity patterns of NR domains. Here we describe the 2.9 Å crystal structure of the multidomain human HNF-4α homodimer bound to its DNA response element and coactivator-derived peptides. A convergence zone connects multiple receptor domains in an asymmetric fashion, joining distinct elements from each monomer. An arginine target of PRMT1 methylation protrudes directly into this convergence zone and sustains its integrity. A serine target of protein kinase C is also responsible for maintaining domain–domain interactions. These post-translational modifications lead to changes in DNA binding by communicating through the tightly connected surfaces of the quaternary fold. We find that some MODY1 mutations, positioned on the ligand-binding domain and hinge regions of the receptor, compromise DNA binding at a distance by communicating through the interjunctional surfaces of the complex. The overall domain representation of the HNF-4α homodimer is different from that of the PPAR-γ–RXR-α heterodimer, even when both NR complexes are assembled on the same DNA element. Our findings suggest that unique quaternary folds and interdomain connections in NRs could be exploited by small-molecule allosteric modulators that affect distal functions in these polypeptides.

Structure of the heterodimeric Ecdysone Receptor DNA-binding complex

Ecdysteroids initiate molting and metamorphosis in insects via a heterodimeric receptor consisting of the ecdysone receptor (EcR) and ultraspiracle (USP). The EcR–USP heterodimer preferentially mediates transcription through highly degenerate pseudo‐palindromic response elements, resembling inverted repeats of 5′‐AGGTCA‐3′ separated by 1 bp (IR‐1). The requirement for a heterodimeric arrangement of EcR–USP subunits to bind to a symmetric DNA is unusual within the nuclear receptor superfamily. We describe the 2.24 Å structure of the EcR–USP DNA‐binding domain (DBD) heterodimer bound to an idealized IR‐1 element. EcR and USP use similar surfaces, and rely on the deformed minor groove of the DNA to establish protein–protein contacts. As retinoid X receptor (RXR) is the mammalian homolog of USP, we also solved the 2.60 Å crystal structure of the EcR–RXR DBD heterodimer on IR‐1 and found the dimerization and DNA‐binding interfaces to be the same as in the EcR–USP complex. Sequence alignments indicate that the EcR–RXR heterodimer is an important model for understanding how the FXR–RXR heterodimer binds to IR‐1 sites.

Large-scale evaluation of protein reductive methylation for improving protein crystallization

Large-scale evaluation of protein reductive methylation for improving protein crystallization

Small-Molecule Scaffolds for CYP51 Inhibitors Identified by High-Throughput Screening and Defined by X-Ray Crystallography

ABSTRACT Sterol 14α-demethylase (CYP51), a major checkpoint in membrane sterol biosynthesis, is a key target for fungal antibiotic therapy. We sought small organic molecules for lead candidate CYP51 inhibitors. The changes in CYP51 spectral properties following ligand binding make CYP51 a convenient target for high-throughput screening technologies. These changes are characteristic of either substrate binding (type I) or inhibitor binding (type II) in the active site. We screened a library of 20,000 organic molecules against Mycobacterium tuberculosis CYP51 (CYP51Mt), examined the top type I and type II binding hits for their inhibitory effects on M. tuberculosis in broth culture, and analyzed them spectrally for their ability to discriminate between CYP51Mt and two reference M. tuberculosis CYP proteins, CYP130 and CYP125. We determined the binding mode for one of the top type II hits, α-ethyl-N-4-pyridinyl-benzeneacetamide (EPBA), by solving the X-ray structure of the CYP51Mt-EPBA complex to a resolution of 1.53 Å. EPBA binds coordinately to the heme iron in the CYP51Mt active site through a lone pair of nitrogen electrons and also through hydrogen bonds with residues H259 and Y76, which are invariable in the CYP51 family, and hydrophobic interactions in a phylum- and/or substrate-specific cavity of CYP51. We also identified a second compound with structural and binding properties similar to those of EPBA, 2-(benzo[d]-2,1,3-thiadiazole-4-sulfonyl)-2-amino-2-phenyl-N-(pyridinyl-4)-acetamide (BSPPA). The congruence between the geometries of EPBA and BSPPA and the CYP51 binding site singles out EPBA and BSPPA as lead candidate CYP51 inhibitors with optimization potential for efficient discrimination between host and pathogen enzymes.