Bloom's syndrome is an autosomal recessive genome-instability disorder associated with a predisposition to cancer, premature aging and developmental abnormalities. It is caused by mutations that inactivate the DNA helicase activity of the BLM protein or nullify protein expression. The BLM helicase has been implicated in the alternative lengthening of telomeres (ALT) pathway, which is essential for the limitless replication of some cancer cells. This pathway is used by 10-15% of cancers, where inhibitors of BLM are expected to facilitate telomere shortening, leading to apoptosis or senescence. Here, the crystal structure of the human BLM helicase in complex with ADP and a 3'-overhang DNA duplex is reported. In addition to the helicase core, the BLM construct used for crystallization (residues 640-1298) includes the RecQ C-terminal (RQC) and the helicase and ribonuclease D C-terminal (HRDC) domains. Analysis of the structure provides detailed information on the interactions of the protein with DNA and helps to explain the mechanism coupling ATP hydrolysis and DNA unwinding. In addition, mapping of the missense mutations onto the structure provides insights into the molecular basis of Bloom's syndrome.

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Interleukin-2 tyrosine kinase, Itk, is an important member of the Tec family of non-receptor tyrosine kinases that play a central role in signaling through antigen receptors such as the T-cell receptor, B-cell receptor, and Fc⑀. Selective inhibition of Itk may be an important way of modulating many diseases involving heightened or inappropriate activation of the immune system. In addition to an unliganded nonphophorylated Itk catalytic kinase domain, we determined the crystal structures of the phosphorylated and nonphosphorylated kinase domain bound to staurosporine, a potent broad-spectrum kinase inhibitor. These structures are useful for the design of novel, highly potent and selective Itk inhibitors and provide insight into the influence of inhibitor binding and phosphorylation on the conformation of Itk.The Tec kinases are a family of five non-receptor tyrosine kinases that play a central role in signaling through antigenreceptors such as the T-cell receptor (TCR), 1 B-cell receptor, and Fc⑀ (1) and are essential for T-cell activation. Three members of the family, Itk, Rlk, and Tec, are activated downstream of antigen receptor engagement in T-cells and transmit signals to downstream effectors, including PLC-␥. A fourth member, Btk, appears to act independently of T-cell signaling and is essential for B-cell development and activation. Btk-deficient murine mast cells have reduced degranulation and decreased production of proinflammatory cytokines following Fc⑀RI crosslinking (2). Btk deletion in mice has a profound effect on B-cell proliferation induced by anti-IgM and inhibits immune responses to thymus-independent type II antigens (3, 4). A biological role for the final member of this family, Bmx, has not been identified.Itk is a key member of this family, and a number of factors point to the importance of this kinase in immune disease.Deletion of Itk in mice results in reduced TCR-induced proliferation and secretion of the cytokines IL-2, IL-4, IL-5, IL-10, and interferon-␥ (5-7). Also, the immunological symptoms of allergic asthma are attenuated in ItkϪ/Ϫ mice, and lung inflammation, eosinophil infiltration, and mucous production are drastically reduced in response to challenge with the allergen OVA (8). Furthermore, the Itk gene is reported to be more highly expressed in peripheral blood T-cells from patients with moderate or severe atopic dermatitis than in controls or patients with mild atopic dermatitis (9).In certain cell types, the role of Itk may be intricately linked with other members of the family. For example, in mast cells, Btk and Itk are both expressed and activated by Fc⑀RI crosslinking (10). Splenocytes from RlkϪ/Ϫ mice secrete half the IL-2 produced by wild type animals in response to TCR engagement (5), whereas the combined deletion of Itk and Rlk in mice leads to a profound inhibition of TCR-induced responses, including proliferation and production of the cytokines IL-2, IL-4, IL-5, and interferon-␥ (5, 7). Furthermore, intracellular signaling following TCR engagement is affected in Itk...

The TIM10 complex is localized in the mitochondrial intermembrane space and mediates insertion of hydrophobic proteins at the inner membrane. We have characterized TIM10 assembly and analyzed the structural properties of its subunits, Tim9 and Tim10. Both proteins are ␣-helical with a protease-resistant central domain, and each self-associates to form mainly dimers and trimers in solution. Tim9 and Tim10 bound to one another with submicromolar affinity in equimolar amounts and assembled in a stable, significantly extended complex that was indistinguishable from the native mitochondrial TIM10 complex. Importantly, the reconstituted TIM10 complex is functional because it bound to the physiological substrate ADP/ATP carrier and displayed chaperone activity in refolding the model substrate firefly luciferase. These data demonstrate that the individual subunits can exist as independent, dynamically self-associating proteins. Assembly into the thermodynamically stable hexameric complex is necessary for the TIM10 chaperone function.Almost all mitochondrial proteins are synthesized in the cytosol and then imported into the organelle in a process that is dictated by each protein's sequence and that is ensured by the function of specialized translocation machineries in the organelle (1-4). Most import and intramitochondrial sorting pathways are variants of the general "matrix pathway," first described in detail for matrix-targeted proteins (1-4). In this pathway, a mitochondrial precursor, which is usually synthesized with an N-terminal, positively charged, amphiphilic presequence, first interacts with cytosolic chaperones. It is then bound by a hetero-oligomeric receptor system on the surface of mitochondria in a process that requires ATP hydrolysis in the cytosol. The polypeptide chain is then transported across two hetero-oligomeric protein import channels, the TOM complex in the outer membrane and the TIM23 complex in the inner membrane. Translocation is completed by the electrophoretic function of the electrochemical potential across the inner membrane and the ATP-powered import motor attached to the inner side of the TIM23 complex. In this pathway, targeting of the polypeptide chain from one complex to the other appears to be directed by increasing avidity of the positive presequence for a series of acidic receptor domains ("acid chain hypothesis") (5-9).The structural basis of this mechanism is becoming increasingly clear through advances in understanding the structural characteristics of key components at different steps of this pathway. First, the solution structure (determined by NMR) of the cytosolic part of the Tom20 receptor in complex with a synthetic presequence has been solved (10, 11): this showed that the presequence is in ␣-helical conformation and that binding between the receptor and the presequence involves hydrophobic stretches. Second, the existence of a hydrophilic channel of TOM40 as the outer membrane import pore and its dynamic behavior have been established (12-16). Third, Tim23 was shown to f...

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress against GS regulatory helices. This keeps GS in an inactive conformation mediated by phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided mutagenesis perturbing interactions with phosphorylated tails led to increased basal/unstimulated GS activity. We propose that multivalent phosphorylation supports GS autoinhibition through interactions from a dynamic “spike” region, allowing a tuneable rheostat for regulating GS activity. This work therefore provides insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.

MurG is an essential bacterial glycosyltransferase enzyme in Pseudomonas aeruginosa performing one of the key membrane steps of peptidoglycan synthesis catalyzing the transfer of N-acetyl glucosamine (GlcNAc) from its donor substrate, UDP-GlcNAc, to the acceptor substrate Lipid I. We have solved the crystal structure of the complex between Pseudomonas aeruginosa MurG and UDP-GlcNAc and compared it with the previously solved complex from E. coli. The structure reveals a large-scale conformational change in the relative orientations of the N- and C-terminal domains, which has the effect of widening the cofactor binding site and displacing the UDP-GlcNAc donor. These results suggest new opportunities to design potent inhibitors of peptidoglycan biosynthesis.

The protein import pathway that targets proteins to the mitochondrial matrix has been extensively characterized in the past 15 years. Variations of this import pathway account for the sorting of proteins to other compartments as well, but the insertion of integral inner membrane proteins lacking a presequence is mediated by distinct translocation machinery. This consists of a complex of Tim9 and Tim10, two homologous, Zn(2+)-binding proteins that chaperone the passage of the hydrophobic precursor across the aqueous intermembrane space. The precursor is then targeted to another, inner-membrane-bound, complex of at least five subunits that facilitates insertion. Biochemical and genetic experiments have identified the key components of this process; we are now starting to understand the molecular mechanism. This review highlights recent advances in this new membrane protein insertion pathway.

The protein import pathway that targets proteins to the mitochondrial matrix has been extensively characterized in the past 15 years. Variations of this import pathway account for the sorting of proteins to other compartments as well, but the insertion of integral inner membrane proteins lacking a presequence is mediated by distinct translocation machinery. This consists of a complex of Tim9 and Tim10, two homologous, Zn(2+)-binding proteins that chaperone the passage of the hydrophobic precursor across the aqueous intermembrane space. The precursor is then targeted to another, inner-membrane-bound, complex of at least five subunits that facilitates insertion. Biochemical and genetic experiments have identified the key components of this process; we are now starting to understand the molecular mechanism. This review highlights recent advances in this new membrane protein insertion pathway.