J. Appl. Cryst.の発刊に際して

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

TGF-beta signal transduction.

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Mitochondrial Membrane Permeabilization in Cell Death

Caspases : intracellular signaling by proteolysis

In the last decade, basic cancer research has produced remarkable advances in our understanding of cancer biology and cancer genetics. Among the most important of these advances is the realization that apoptosis and the genes that control it have a profound effect on the malignant phenotype. For example, it is now clear that some oncogenic mutations disrupt apoptosis, leading to tumor initiation, progression or metastasis. Conversely, compelling evidence indicates that other oncogenic changes promote apoptosis, thereby producing selective pressure to override apoptosis during multistage carcinogenesis. Finally, it is now well documented that most cytotoxic anticancer agents induce apoptosis, raising the intriguing possibility that defects in apoptotic programs contribute to treatment failure. Because the same mutations that suppress apoptosis during tumor development also reduce treatment sensitivity, apoptosis provides a conceptual framework to link cancer genetics with cancer therapy. An intense research effort is uncovering the underlying mechanisms of apoptosis such that, in the next decade, one envisions that this information will produce new strategies to exploit apoptosis for therapeutic benefit.

Apoptosis in cancer

Pattern lormation in the dorsal region of the Drosophila embryo depends on the activity of a small group of zygotically acting genes, dpp, a key gene in this group, encodes a TGF-p-like product (Dpp) that has been proposed to function as a morphogen with peak levels of Dpp-specifying amnioserosa, the dorsal-most cell type, and lower Dpp levels specifying dorsal ectoderm. The short gastrulation gene also contributes to patterning the dorsal region, but unlike the other genes involved in this process, sog activity is only required in ventral cells. Genetic evidence indicates that sog functions to antagonize dpp activity. In this report we present further phenotypic characterization of sog mutant embryos in dorsal and lateral regions and describe the cloning of the sog locus, sog is expressed in a broad lateral stripe of cells that abuts the dorsal territory of dfpp-expressing cells, sog is predicted to encode a protein with an internal signal sequence and a large extracellular domain containing four repeats of a novel motif defined by the spacing of 10 cysteine residues that is distantly related to domains present in thrombospondin and procollagen. We propose that one or more of these cysteine repeats can be liberated by proteolytic cleavage of the primary Sog protein. These putative soluble Sog peptides may then diffuse into the dorsal region to antagonize the activity of Dpp, leading to the subdivision of the dorsal territory into amnioserosa and dorsal ectoderm.

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Dorsal-ventral patterning of the Drosophila embryo depends on a putative negative growth factor encoded by the short gastrulation gene.

Enforced expression of the antiapoptotic Bcl-2 family protein Mcl-1 promotes lymphomagenesis in the mouse; however, the functional role of Mcl-1 in human B-cell lymphoma remains unclear. We demonstrate that Mcl-1 is widely expressed in malignant B-cells, and high-level expression of Mcl-1 is required for B-lymphoma cell survival, since transfection of Mcl-1-specific antisense oligodeoxynucleotides was sufficient to promote apoptosis in Akata6 lymphoma cells. Mcl-1 was efficiently cleaved by caspases at evolutionarily conserved aspartic acid residues in vitro, and during cisplatin-induced apoptosis in B-lymphoma cell lines and spontaneous apoptosis of primary malignant B-cells. Overexpression of the Mcl-1 cleavage product that accumulated during apoptosis was sufficient to kill cells. Therefore, Mcl-1 is an essential survival molecule for B-lymphoma cells and is cleaved by caspases to a death-promoting molecule during apoptosis. In contrast to Mcl-1, Bcl-2 and Bcl-XL were relatively resistant to caspase cleavage in vitro and in intact cells. Interfering with Mcl-1 function appears to be an effective means of inducing apoptosis in Mcl-1-positive B-cell lymphoma, and the unique sensitivity of Mcl-1 to caspase-mediated cleavage suggests an attractive strategy for converting it to a proapoptotic molecule.

Mcl-1 is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule by efficient caspase-mediated cleavage

Carbonic anhydrases fall into three distinct evolutionary and structural classes: α, β, and γ. The β-class carbonic anhydrases (β-CAs) are widely distributed among higher plants, simple eukaryotes, eubacteria, and archaea. We have determined the crystal structure of ECCA, a β-CA from Escherichia coli, to a resolution of 2.0 A. In agreement with the structure of the β-CA from the chloroplast of the red alga Porphyridium purpureum, the active-site zinc in ECCA is tetrahedrally coordinated by the side chains of four conserved residues. These results confirm the observation of a unique pattern of zinc ligation in at least some β-CAs. The absence of a water molecule in the inner coordination sphere is inconsistent with known mechanisms of CA activity. ECCA activity is highly pH-dependent in the physiological range, and its expression in yeast complements an oxygen-sensitive phenotype displayed by a β-CA-deletion strain. The structural and biochemical characterizations of ECCA presented here and the comparisons with other β-CA structures suggest that ECCA can adopt two distinct conformations displaying widely divergent catalytic rates.

Jason W. O'Neill

Papers

Dorsal-ventral patterning of the Drosophila embryo depends on a putative negative growth factor encoded by the short gastrulation gene.

Mcl-1 is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule by efficient caspase-mediated cleavage

Crystal structure of E. coli beta-carbonic anhydrase, an enzyme with an unusual pH-dependent activity.

Mitochondria and apoptosis: new therapeutic targets.

BCL-XL dimerization by three-dimensional domain swapping.