2017-03-22Single-wavelength anomalous dispersion (SAD) utilizing anomalous signal from native S atoms, or other atoms with Z ≤ 20, generally requires highly redundant data collected using relatively long-wavelength X-rays. Here, the results from two proteins are presented where the anomalous signal from serendipitously acquired surface-bound Ca atoms with an anomalous data multiplicity of around 10 was utilized to drive de novo structure determination. In both cases, the Ca atoms were acquired from the crystallization solution, and the data-collection strategy was not optimized to exploit the anomalous signal from these scatterers. The X-ray data were collected at 0.98 Å wavelength in one case and at 1.74 Å in the other (the wavelength was optimized for sulfur, but the anomalous signal from calcium was exploited for structure solution). Similarly, using a test case, it is shown that data collected at ∼1.0 Å wavelength, where the f′′ value for sulfur is 0.28 e, are sufficient for structure determination using intrinsic S atoms from a strongly diffracting crystal. Interestingly, it was also observed that SHELXD was capable of generating a substructure solution from high-exposure data with a completeness of 70% for low-resolution reflections extending to 3.5 Å resolution with relatively low anomalous multiplicity. Considering the fact that many crystallization conditions contain anomalous scatterers such as Cl, Ca, Mn etc., checking for the presence of fortuitous anomalous signal in data from well diffracting crystals could prove useful in either determining the structure de novo or in accurately assigning surface-bound atoms.
2017-03-22The serine/threonine protein kinase liver kinase B1 (LKB1) is a tumour suppressor and plays important roles in development and metabolism. It phosphorylates AMPK and AMPK-related kinases to regulate multiple physiological processes. Mutations in LKB1 often occur in multiple cancers. LKB1 can be suppressed by 14-3-3 proteins in a phosphorylation-dependent manner. Previously, the structure of a 14-3-3ζ–LKB1 fusion protein has been reported, revealing a phosphorylation-independent binding mode of LKB1 to 14-3-3 proteins. Here, the crystal structure of phosphorylated LKB1 peptide in complex with 14-3-3ζ was solved, which provides a structural basis for the phosphorylation-dependent recognition of LKB1 by 14-3-3 proteins.
2017-03-22Adherence to host cells is a crucial step in the process of bacterial infection, which is usually mediated by a number of outer membrane proteins identified as adhesins. Helicobacter pylori adhesin A (HpaA) is a member of the adhesin family that mediates the adherence of Helicobacter pylori to gastric epithelial cells, and consequently assists the bacteria in becoming a life-long colonizer of the human stomach. In this study, two constructs were made for the production of truncated HpaA proteins comprising residues 31–260 and 53–260, respectively. The products of both constructs were crystallized, but only the protein from the shorter construct (residues 53–260) formed crystals that were capable of diffraction. In the subsequent optimization trials, crystals in different forms were unexpectedly obtained by using lithium sulfate and ammonium sulfate as the precipitant. An X-ray data set was collected to 1.95 Å resolution on beamline BL18U1 at SSRF using a crystal grown with 1.92 M lithium sulfate, which belonged to space group P65 with unit-cell parameters a = b = 95.42, c = 54.72 Å, γ = 120°, while another crystal grown with 1.9 M ammonium sulfate diffracted to 2.60 Å resolution and the collected data set was indexed in space group P21212, with unit-cell parameters a = 121.01, b = 190.56, c = 106.31 Å. The collection of diffraction data has established a solid basis for structure determination.
2017-03-22The heterogeneous ribonucleoprotein A18 (hnRNP A18) is upregulated in hypoxic regions of various solid tumors and promotes tumor growth via the coordination of mRNA transcripts associated with pro-survival genes. Thus, hnRNP A18 represents an important therapeutic target in tumor cells. Presented here is the first X-ray crystal structure to be reported for the RNA-recognition motif of hnRNP A18. By comparing this structure with those of homologous RNA-binding proteins (i.e. hnRNP A1), three residues on one face of an antiparallel β-sheet (Arg48, Phe50 and Phe52) and one residue in an unstructured loop (Arg41) were identified as likely to be involved in protein–nucleic acid interactions. This structure helps to serve as a foundation for biophysical studies of this RNA-binding protein and structure-based drug-design efforts for targeting hnRNP A18 in cancer, such as malignant melanoma, where hnRNP A18 levels are elevated and contribute to disease progression.
2017-03-22S100A1 is a member of the S100 family of Ca2+-binding proteins and regulates several cellular processes, including those involved in Ca2+ signaling and cardiac and skeletal muscle function. In Alzheimer's disease, brain S100A1 is overexpressed and gives rise to disease pathologies, making it a potential therapeutic target. The 2.25 Å resolution crystal structure of Ca2+-S100A1 is solved here and is compared with the structures of other S100 proteins, most notably S100B, which is a highly homologous S100-family member that is implicated in the progression of malignant melanoma. The observed structural differences in S100A1 versus S100B provide insights regarding target protein-binding specificity and for targeting these two S100 proteins in human diseases using structure-based drug-design approaches.
2017-03-22Citrus variegated chlorosis is a disease that attacks economically important citrus plantations and is caused by the plant-pathogenic bacterium Xylella fastidiosa. In this work, the structure of a small heat-shock protein from X. fastidiosa (XfsHSP17.9) is reported. The high-order structures of small heat-shock proteins from other organisms are arranged in the forms of double-disc, hollow-sphere or spherical assemblies. Unexpectedly, the structure reported here reveals a high-order architecture forming a nearly square cavity.
2017-03-22Enolase is an important enzyme in glycolysis and various biological processes. Its dysfunction is closely associated with diseases. Here, the enolase from Drosophila melanogaster (DmENO) was purified and crystallized. A crystal of DmENO diffracted to 2.0 Å resolution and belonged to space group R32. The structure was solved by molecular replacement. Like most enolases, DmENO forms a homodimer with conserved residues in the dimer interface. DmENO possesses an open conformation in this structure and contains conserved elements for catalytic activity. This work provides a structural basis for further functional and evolutionary studies of enolase.