Ionic strength and pH dependence of the reaction between plastocyanin and Photosystem 1. Evidence of a rate-limiting conformational change

The electron-transfer reaction between spinach wild-type plastocyanin (Pc(WT)) two site-directed mutants, Pc(Thr79His) and Pc(Lys81His), and spinach Photosystem 1 particles, has been studied as a function of protein concentration, ionic strength and pH by using laser-flash absorption spectroscopy. The kinetic data are interpreted using the simplest possible three-step model, involving a rate-limiting conformational change preceding intracomplex electron transfer. The three proteins show similar concentration, pH and ionic strength dependencies. The effects of ionic strength and pH on the reaction indicate a strong influence of complementary charges on complex formation and stabilization. Studies with apoprotein support the opinion that the hydrophobic patch is critical for an productive interaction with the reaction center of Photosystem 1. Together with earlier site-directed mutagenesis studies, the absence of a detectable Photosystem 1 reaction in the presence of reduced azurin, stellacyanin, cytochrome c and cytochrome c₅₅₁, demonstrates the existence of a high level of specificity in the protein-protein interface in the formation of an efficient electron-transfer complex.     

Structural genomics efforts at the Chinese Academy of Sciences and Peking University

Structural genomics efforts at the Chinese Academy of Sciences and Peking University are reported in this article. The major targets for the structural genomics project are targeted proteins expressed in human hematopoietic stem/progenitor cells, proteins related to blood diseases and other human proteins. Up to now 328 target genes have been constructed in expression vectors. Among them, more than 50% genes have been expressed in Escherichia coli, approximately 25% of the resulting proteins are soluble, and 35 proteins have been purified. Crystallization, data collection and structure determination are continuing. Experiences accumulated during this initial stage are useful for designing and applying high-throughput approaches in structural genomics.Abbreviations: NSFC, National Natural Science foundation of China; MOST, Ministry of Science and Technology of China; CAS, Chinese Academy of Sciences; NSRL, National Synchrotron Radiation Laboratory in Hefei; BSRF, Beijing Synchrotron Radiation Facilities; HSPC, Hematopoietic stem/progenitor cells; APL, acute promyelocytic leukemia; ATRA, all-trans retinoic acid; COG, Cluster of Orthologous Groups of proteins.     

Structural and Functional Characterization of the JH2 Pseudokinase Domain of JAK Family Tyrosine Kinase 2 (TYK2)

JAK (Janus family of cytoplasmic tyrosine kinases) family tyrosine kinase 2 (TYK2) participates in signaling through cytokine receptors involved in immune responses and inflammation. JAKs are characterized by dual kinase domain: a tyrosine kinase domain (JH1) that is preceded by a pseudokinase domain (JH2). The majority of disease-associated mutations in JAKs map to JH2, demonstrating its central regulatory function. JH2s were considered catalytically inactive, but JAK2 JH2 was found to have low autoregulatory catalytic activity. Whether the other JAK JH2s share ATP binding and enzymatic activity has been unclear. Here we report the crystal structure of TYK2 JH2 in complex with adenosine 5′-O-(thiotriphosphate) (ATP-γS) and characterize its nucleotide binding by biochemical and biophysical methods. TYK2 JH2 did not show phosphotransfer activity, but it binds ATP and the nucleotide binding stabilizes the protein without inducing major conformational changes. Mutation of the JH2 ATP-binding pocket increased basal TYK2 phosphorylation and downstream signaling. The overall structural characteristics of TYK2 JH2 resemble JAK2 JH2, but distinct stabilizing molecular interactions around helix αAL in the activation loop provide a structural basis for differences in substrate access and catalytic activities among JAK family JH2s. The structural and biochemical data suggest that ATP binding is functionally important for both TYK2 and JAK2 JH2s, whereas the regulatory phosphorylation appears to be a unique property of JAK2. Finally, the co-crystal structure of TYK2 JH2 complexed with a small molecule inhibitor demonstrates that JH2 is accessible to ATP-competitive compounds, which offers novel approaches for targeting cytokine signaling as well as potential therapeutic applications.     

Engineered variants of InlB with an additional leucine‐rich repeat discriminate between physiologically relevant and packing contacts in crystal structures of the InlB:MET complex

The physiological relevance of contacts in crystal lattices often remains elusive. This was also the case for the complex between the invasion protein internalin B (InlB) from Listeria monocytogenes and its host cell receptor, the human receptor tyrosine kinase (RTK) MET. InlB is a MET agonist and induces bacterial host cell invasion. Activation of RTKs generally involves ligand‐induced dimerization of the receptor ectodomain. The two currently available crystal structures of the InlB:MET complex show the same arrangement of InlB and MET in a 1:1 complex, but different dimeric 2:2 assemblies. Only one of these 2:2 assemblies is predicted to be stable by a computational procedure. This assembly is mainly stabilized by a contact between the Cap domain of InlB from one and the Sema domain of MET from another 1:1 complex. Here, we probe the physiological relevance of this interaction. We generated variants of the leucine‐rich repeat (LRR) protein InlB by inserting an additional repeat between the first and the second LRR. This should allow formation of the 1:1 complex but disrupt the potential 2:2 complex involving the Cap‐Sema contact due to steric distortions. A crystal structure of one of the engineered proteins showed that it folded properly. Binding affinity to MET was comparable to that of wild‐type InlB. The InlB variant induced MET phosphorylation and cell scatter like wild‐type InlB. These results suggest that the Cap‐Sema interaction is not physiologically relevant and support the previously proposed assembly, in which a 2:2 InlB:MET complex is built around a ligand dimer.