Sunggeon Ko

Structural basis of E2-25K/UBB+1 interaction leading to proteasome inhibition and neurotoxicity.

E2–25K/Hip2 is an unusual ubiquitin-conjugating enzyme that interacts with the frameshift mutant of ubiquitin B (UBB+1) and has been identified as a crucial factor regulating amyloid-β neurotoxicity. To study the structural basis of the neurotoxicity mediated by the E2–25K-UBB+1 interaction, we determined the three-dimensional structures of UBB+1, E2–25K and the E2–25K/ubiquitin, and E2–25K/UBB+1 complex. The structures revealed that ubiquitin or UBB+1 is bound to E2–25K via the enzyme MGF motif and residues in α9 of the enzyme. Polyubiquitylation assays together with analyses of various E2–25K mutants showed that disrupting UBB+1 binding markedly diminishes synthesis of neurotoxic UBB+1-anchored polyubiquitin. These results suggest that the interaction between E2–25K and UBB+1 is critical for the synthesis and accumulation of UBB+1-anchored polyubiquitin, which results in proteasomal inhibition and neuronal cell death.

Structural and Cell Adhesion Properties of Zebrafish Syndecan-4 Are Shared with Higher Vertebrates

The syndecan proteoglycans are an ancient class of receptor, bearing heparan sulfate chains that interact with numerous potential ligands including growth factors, morphogens, and extracellular matrix molecules. The single syndecan of invertebrates appears not to have cell adhesion roles, but these have been described for mammalian paralogues, especially syndecan-4. This member is best understood in terms of interactions, signaling, and structure of its cytoplasmic domain. The zebrafish homologue of syndecan-4 has been genetically linked to cell adhesion and migration in zebrafish embryos, but no molecular and cellular studies have been reported. Here it is demonstrated that key functional attributes of syndecan-4 are common to both zebrafish and mammalian homologues. These include glycosaminoglycan substitution, a NXIP motif in the extracellular domain that promotes integrin-mediated cell adhesion, and a transmembrane GXXXG motif that promotes dimer formation. In addition, despite some amino acid substitutions in the cytoplasmic domain, its ability to form twisted clamp dimers is preserved, as revealed by nuclear magnetic resonance spectroscopy. This technique also showed that phosphatidylinositol 4,5-bisphosphate can interact with the zebrafish syndecan-4 cytoplasmic domain, and that the molecule in its entirety supports focal adhesion formation, and complements the murine null cells to restore a normal actin cytoskeleton identically to the rat homologue. Therefore, the cell adhesion properties of syndecan-4 are consistent across the vertebrate spectrum and reflect an early acquisition of specialization after syndecan gene duplication events at the invertebrate/early chordate boundary.

Structure of the DNA-binding domain of NgTRF1 reveals unique features of plant telomere-binding proteins

Telomeres are protein–DNA elements that are located at the ends of linear eukaryotic chromosomes. In concert with various telomere-binding proteins, they play an essential role in genome stability. We determined the structure of the DNA-binding domain of NgTRF1, a double-stranded telomere-binding protein of tobacco, using multidimensional NMR spectroscopy and X-ray crystallography. The DNA-binding domain of NgTRF1 contained the Myb-like domain and C-terminal Myb-extension that is characteristic of plant double-stranded telomere-binding proteins. It encompassed amino acids 561–681 (NgTRF1561–681), and was composed of 4 α-helices. We also determined the structure of NgTRF1561–681 bound to plant telomeric DNA. We identified several amino acid residues that interacted directly with DNA, and confirmed their role in the binding of NgTRF1 to telomere using site-directed mutagenesis. Based on a structural comparison of the DNA-binding domains of NgTRF1 and human TRF1 (hTRF1), NgTRF1 has both common and unique DNA-binding properties. Interaction of Myb-like domain with telomeric sequences is almost identical in NgTRF1561–681 with the DNA-binding domain of hTRF1. The interaction of Arg-638 with the telomeric DNA, which is unique in NgTRF1561–681, may provide the structural explanation for the specificity of NgTRF1 to the plant telomere sequences, (TTTAGGG)n.

Crystal structure of Fushi tarazu factor 1 ligand binding domain/Fushi tarazu peptide complex identifies new class of nuclear receptors.

The interaction between the orphan nuclear receptor FTZ-F1 (Fushi tarazu factor 1) and the segmentation gene protein FTZ is critical for specifying alternate parasegments in the Drosophila embryo. Here, we have determined the structure of the FTZ-F1 ligand-binding domain (LBD)·FTZ peptide complex using x-ray crystallography. Strikingly, the ligand-binding pocket of the FTZ-F1 LBD is completely occupied by helix 6 (H6) of the receptor, whereas the cofactor FTZ binds the co-activator cleft site of the FTZ-F1 LBD. Our findings suggest that H6 is essential for transcriptional activity of FTZ-F1; this is further supported by data from mutagenesis and activity assays. These data suggest that FTZ-F1 might belong to a novel class of ligand-independent nuclear receptors. Our findings are intriguing given that the highly homologous human steroidogenic factor-1 and liver receptor homolog-1 LBDs exhibit sizable ligand-binding pockets occupied by putative ligand molecules.

Solution structure of the DNA binding domain of rice telomere binding protein RTBP1.

RTBP1 is a rice telomeric protein that binds to the duplex array of TTTAGGG repeats at chromosome ends. The DNA binding domain of RTBP1 contains a Myb-type DNA binding motif and a highly conserved C-terminal Myb extension that is unique to plant telomeric proteins. Using an electrophoretic mobility shift assay, we identified the C-terminal 110-amino acid region (RTBP1(506-615)) as the minimal telomeric DNA binding domain, suggesting that the Myb extension is required for binding plant telomeric DNA. Like other telomeric proteins such as human TRF1 and yeast Rap1, RTBP1 induced a DNA bending in the telomeric repeat sequence, suggesting that RTBP1 may play a role in establishing and/or maintaining an active telomere configuration in vivo. To elucidate the DNA binding mode of RTBP1, we determined the three-dimensional structure of RTBP1(506-615) in solution by NMR spectroscopy. The overall structure of RTBP1(506-615) is composed of four alpha-helices and stabilized by three hydrophobic patches. The second and third helices in RTBP1 form a helix-turn-helix motif that interacts directly with DNA. The fourth helix located in the Myb extension is essential for binding to telomeric DNA via stabilization of the overall structure of the RTBP1 DNA binding domain. When DNA bound to RTBP1(506-615), large chemical shift perturbations were induced in the N-terminal arm, helix 3, and the loop between helices 3 and 4. These results suggest that helix 3 functions as a sequence-specific recognition helix while the N-terminal arm stabilizes the DNA binding.

Crystal Structures and Enzyme Mechanisms of a Dual Fucose Mutarotase/Ribose Pyranase

Escherichia coli FucU (Fucose Unknown) is a dual fucose mutarotase and ribose pyranase, which shares 44% sequence identity with its human counterpart. Herein, we report the structures of E. coli FucU and mouse FucU bound to L-fucose and delineate the catalytic mechanisms underlying the interconversion between stereoisomers of fucose and ribose. E. coli FucU forms a decameric toroid with each active site formed by two adjacent subunits. While one subunit provides most of the fucose-interacting residues including a catalytic tyrosine residue, the other subunit provides a catalytic His-Asp dyad. This active-site feature is critical not only for the mutarotase activity toward L-fucose but also for the pyranase activity toward D-ribose. Structural and biochemical analyses pointed that mouse FucU assembles into four different oligomeric forms, among which the smallest homodimeric form is most abundant and would be the predominant species under physiological conditions. This homodimer has two fucose-binding sites that are devoid of the His-Asp dyad and catalytically inactive, indicating that the mutarotase and the pyranase activities appear dispensable in vertebrates. The defective assembly of the mouse FucU homodimer into the decameric form is due to an insertion of two residues at the N-terminal extreme, which is a common aspect of all the known vertebrate FucU proteins. Therefore, vertebrate FucU appears to serve for as yet unknown function through the quaternary structural alteration.

Structural basis of the auto-inhibition mechanism of nonreceptor tyrosine kinase PTK6

Protein tyrosine kinase 6 (PTK6) is composed of SH3, SH2, and Kinase domains, with a linker region (Linker) between the SH2 and Kinase domains. Here, we report the structural basis of the SH3-Linker interaction that results in auto-inhibition of PTK6. The solution structures of the SH3 domain and SH3/Linker complex were determined by NMR spectroscopy. The structure of the SH3 domain forms a conventional beta-barrel with two beta-sheets comprised of five beta-strands. However, the molecular topology and charge distribution of PTK6-SH3 slightly differs from that of the other SH3 domains. The structure of the N-terminal Linker within the complex showed that the proline-rich region (P175-P187) of the Linker forms a compact hairpin structure through hydrophobic interactions. The structure of the SH3/Linker complex revealed intra-molecular interaction between the amino acid pairs R22/E190, W44/W184, N65/P177, and Y66/P179. Mutations in PTK6 at R22, W44, N65, and Y66 residues in the SH3 domain increased catalytic activity compared with wild-type protein, implying that specific interactions between hydrophobic residues in the proline-rich linker region and hydrophobic residues in the SH3 domain are mainly responsible for down-regulating the catalytic activity of PTK6.

Solution structure and Rpn1 interaction of the UBL domain of human RNA polymerase II C-terminal domain phosphatase.

The ubiquitin-like modifier (UBL) domain of ubiquitin-like domain proteins (UDPs) interacts specifically with subunits of the 26 S proteasome. A novel UDP, ubiquitin-like domain-containing C-terminal domain phosphatase (UBLCP1), has been identified as an interacting partner of the 26 S proteasome. We determined the high-resolution solution structure of the UBL domain of human UBLCP1 by nuclear magnetic resonance spectroscopy. The UBL domain of hUBLCP1 has a unique β-strand (β3) and β3-α2 loop, instead of the canonical β4 observed in other UBL domains. The molecular topology and secondary structures are different from those of known UBL domains including that of fly UBLCP1. Data from backbone dynamics shows that the β3-α2 loop is relatively rigid although it might have intrinsic dynamic profile. The positively charged residues of the β3-α2 loop are involved in interacting with the C-terminal leucine-rich repeat-like domain of Rpn1.

Backbone dynamics of an atypical orphan response regulator protein, Helicobacter pylori 1043

An atypical orphan response regulator protein, HP1043 (HP-RR) in Helicobacter pylori, is proven to be essential for cell growth and does not require the well known phosphorelay scheme. HP-RR was identified as a symmetric dimer with two functional domains, an N-terminal regulatory domain (HP-RRr) and a C-terminal effector domain (HP-RRe). HP-RR is a new class of response regulator, as a phosphorylation-independent regulator. Previously, we have presented a detailed three-dimensional structure of HP-RR using NMR spectroscopy and X-ray crystallography. In this study, in order to understand the functional importance of flexibilities in HP-RRr and HP-RRe, T1, T2, heteronuclear NOE experiments have been performed and backbone dynamics of HP-RRr and HP-RRe were investigated. HP-RRr is a symmetric dimer and the interface region, α4-β5-α5 of dimer, showed high rigidity (high S2 values). Site of rearrangements associated with phosphorylation of HP-RRr (Ser75: Rex = 3.382, Ile95: Rex = 5.228) showed slow chemical exchanges. HP-RRe is composed of three α-helices flanked on two sides by anti-parallel β-sheets. Low order parameters as well as conformational exchanges in the centers of loop regions known as the DNA binding site and transcription site of HP-RRe suggested that flexibility of HP-RRe is essential for interaction with DNA. In conclusion, backbone dynamics information for HP-RR implies that structural flexibilities in HP-RRr are necessary for the phosphorylation site and the dynamic nature of HP-RRe is essential for the regulation of interaction between protein and DNA.

Prediction of parathyroid hormone signalling potency using SVMs

Parathyroid hormone is the most important endocrine regulator of calcium concentration. Its N-terminal fragment (1–34) has sufficient activity for biological function. Recently, site-directed mutagenesis studies demonstrated that substitutions at several positions within shorter analogues (1–14) can enhance the bioactivity to greater than that of PTH (1–34). However, designing the optimal sequence combination is not simple due to complex combinatorial problems. In this study, support vector machines were introduced to predict the biological activity of modified PTH (1–14) analogues using mono-substituted experimental data and to analyze the key physicochemical properties at each position that correlated with bioactivity. This systematic approach can reduce the time and effort needed to obtain desirable molecules by bench experiments and provide useful information in the design of simpler activating molecules.