Molecular Replacement Studies of Cucurmosin from Cucurbita Moschata:Structure Homology with Trichosanthin

High diffraction quality crystals of cucurmosin, a type I ribosome inactivating protein isolated from the sarcocarp of Cucurbita moschata (pumpkin), have been grown under newly optimised conditions. With in-house rotating anode X-ray source, these crystals diffract to 1.65 ?resolution which is much higher than that of the previously reported crystals that diffracted only to 3 ?resolution. The crystals belong to space group P212121 with cell parameters a = 41.5, b = 58.4 and c = 99.3 . Molecular replacement studies indicate that the cucurmosin structure is homologous to trichosanthin. The initial structural model has been obtained and the model fitting/ refinement is in progress.

Improvement of a new rotation function for molecular replacement by designing new scoring functions and dynamic correlation coefficient

A previously published new rotation function has been improved by using a dynamic correlation coefficient as well as two new scoring functions of relative entropy and mean-square-residues to make the rotation function more robust and independent of a specific set of weights for scoring and ranking. The previously described new rotation function calculates the rotation function of molecular replacement by matching the search model directly with the Patterson vector map. The signal-to-noise ratio for the correct match was increased by averaging all the matching peaks. Several matching scores were employed to evaluate the goodness of matching. These matching scores were then combined into a single total score by optimizing a set of weights using the linear regression method. It was found that there exists an optimal set of weights that can be applied to the global rotation search and the correct solution can be ranked in the top 100 or less. However, this set of optimal weights in general is dependent on the search models and the crystal structures with different space groups and cell parameters. In this work, we try to solve this problem by designing a dynamic correlation coefficient. It is shown that the dynamic correlation coefficient works for a variety of space groups and cell parameters in the global search of rotation function. We also introduce two new matching scores： relative entropy and mean-square-residues. Last but not least, we discussed a valid method for the optimization of the adjustable parameters for matching vectors.

Crystal Structure of Crotin Ⅱ at 2.5 Resolution

Crotin Ⅱ is one of the ribosome inactivating proteins (RIPs). It belongs to RNA hydrolase. The crystals of crotin Ⅱ are hexagonal with a=b= 94. 62, c=28. 44 A, space group P61, Mr= 14900 Da. The structure was solved by molecularreplacement methed using the molecular structure of RNase T1 as a search model andrefined to R= 0. 25 for the reflections within 10- 2. 5 A resolution range. The refinedmodel cotains one α-helix, one two-strand antiparallel β-sheet and one five-strand an-tiparallel β-sheet. Four conservative residues His47, Glu77, Arg102 and His117 gathering in the cleft of the structure form the possible active site of crotin Ⅱ.

Studies on the Crystal Structure of [L-Met]^(B0) Bovine Insulin at 3.0 Resolution

Based on the crystal symmetry of [L-Met]^(B0) bovine insulin (LMBBI) and the fundamental theory of the molecular packing method, the scheme for the determination of the position and orientation of the molecules by only using one-dimensional rotation and one-dimensional translation was chosen to be used,and therefore the calculation of the rotation function of the molecular replacement method and the refinement of the rotational and translational parameters by using the R-factor search method were simplified greatly. After the preliminary refinement by using the macromolecular rigid body refinement technique, the molecular model was further refined and adjusted by using the energy-minimizing stereochemical-restrained least squares refinement technique assisted by the manual revision on the difference Fourier maps.The L-Met residues on the N-termlnus of the B-chain appeared clearly on the final electron density map.