Crystal structure of the active form of native human thymidylate synthase in the absence of bound substrates.

Human thymidylate synthase (hTS) provides the sole de novo intracellular source of thymidine 5'-monophosphate (dTMP). hTS is required for DNA replication prior to cell division, making it an attractive target for anticancer chemotherapy and drug discovery. hTS binds 2'-deoxyuridine 5'-monophosphate (dUMP) and the folate co-substrate N5,N10-methylenetetrahydrofolate (meTHF) in a pocket near the catalytic residue Cys195. The catalytic loop, which is composed of amino-acid residues 181-197, can adopt two distinct conformations related by a 180° rotation. In the active conformation Cys195 is close to the active site, while in the inactive conformation it is rotated and Cys195 is too distant from the active site for catalysis. Several hTS structures, either native or engineered, have been solved in the active conformation in complex with ligands or inhibitors and at different salt concentrations. However, apo hTS structures have been solved in an inactive conformation in high-salt and low-salt conditions (PDB entries 1ypv, 4h1i, 4gyh, 3egy and 3ehi). Here, the structure of apo hTS crystallized in the active form with sulfate ions coordinated by the arginine residue that binds dUMP is reported.

The human protein HSPC021 interacts with Int-6 and is associated with eukaryotic translation initiation factor 3.

The Int-6 protein has been shown to be a subunit of eukaryotic translation initiation factor 3 (eIF3) and to play a role in the control of cell growth. By immunoprecipitation experiments and mass spectrometry analyses, we identified a human protein previously known as HSPC021 that is associated with Int-6. Exposure of Jurkat cells to the phosphatase inhibitor H(2)O(2) triggers a marked phosphorylation on tyrosine of HSPC021. Several experiments were performed to evaluate whether this protein is associated with eIF3. It was observed that HSPC021 coelutes with Int-6 and eIF3 in gel filtration, coimmunoprecipitates with eIF3, and is incorporated into eIF3 both in rabbit reticulocyte lysates and in COS7 cells. A direct protein-protein interaction occurs between HSPC021 and Int-6, but the analysis of different mutants of HSPC021 indicated that a larger region of the protein is necessary for incorporation into eIF3 as compared with binding to Int-6. Taken together, our results establish that HSPC021 is tightly associated with the mammalian translation initiation factor eIF3. Analysis of the primary sequence of HSPC021 from different species revealed the presence of a tetratricopeptide repeat, a proteasome-COP9 (constitutive photomorphogenesis 9) signalosome-initiation factor 3 domain along with a Pumilio FBF repeat. These protein motifs are also present in subunits of eIF3, of the lid of the 26 S proteasome, and of the COP9 signalosome.

S100-annexin complexes: some insights from structural studies.

Several annexins have been shown to bind proteins that belong to the S100 calcium-binding protein family. The two best-characterized complexes are annexin II with p11 and annexin I with S100C, the former of which has been implicated in membrane fusion processes. We have solved the crystal structures of the complexes of p11 with annexin II N-terminus and of S100C with annexin I N-terminus. Using these structural results, as well as electron microscopy observations of liposome junctions formed in the presence of such complexes (Lambert et al., 1997 J Mol Biol 272, 42-55), we propose a computer generated model for the entire annexin II/p11 complex.