AXIN Shapes Tankyrase ARChitecture.

The poly(ADP-ribose)polymerase (PARP) Tankyrase uses ankyrin repeat modules to capture substrates via Tankyrase-binding peptide motifs. In this issue of Structure, Eisemann et al. (2016) describe how the signaling protein AXIN can access and conformationally adapt the multivalent ankyrin repeat region of Tankyrase and discuss potential implications for enzymatic substrate modification

Regulation of the SRF cofactor MAL by actin

Serum Response Factor (SRF) is controlled by actin dynamics at many of its target genes: Rho-induced depletion of G-actin is sensed by MAL, a member of the myocardin family of SRF coactivators. MAL binds G-actin via its N-terminus, the "RPEL domain", containing three RPEL motifs. MAL rapidly circulates between nucleus and cytoplasm in resting NIH3T3 fibroblasts. It accumulates in the nucleus and activates SRF upon serum stimulation, which alters interactions between G-actin and the RPEL domain. In contrast, myocardin (MC) itself is constitutively nuclear and active when expressed in fibroblasts, suggesting that it is not controlled through Rho. This thesis addresses the mode and functions of actin binding by myocardin-family proteins. Actin binding targets MAL for efficient CRM1-mediated nuclear export. Nuclear accumulation of MAL is not sufficient for activation of SRF-mediated transcription unless an inhibitory MAL-actin interaction in the nucleus is released. Actin therefore fulfils a dual role in MAL regulation by controlling MAL localisation as well as activity. The MAL RPEL domain is sufficient to confer actin-regulated nucleocytoplasmic trafficking and binds multiple actin molecules, efficiently sequestering them from polymerisation. Actin-binding toxins directly interfere with the MAL-actin complex. The RPEL motif represents an actin-binding unit: affinities of MAL RPEL motifs 1 and 2 for actin are relatively high while RPEL3 binds actin weakly. RPEL motifs cooperate to regulate MAL. The regulatory contribution of an RPEL3-actin interaction depends on actin binding by the RPEL 1-2 unit, differences in which account for differential regulation of MAL and MC, which binds actin weakly. A model of MAL regulation by differential actin occupancy of multiple RPEL motifs is proposed. Crystal structures of MAL RPEL motifs 1 and 2 bound to G-actin were obtained. RPEL motifs maintain hydrophobic interactions with a hydrophobic cleft at the subdomain 1-3 interface of actin and a "platform" on subdomain 3, both at the "base" of the actin molecule in its conventional view (Kabsch et al., 1990). The RPEL motif also establishes critical polar interactions with actin. Conservation of the RPEL motif reflects actin binding. The structures rationalise RPEL-actin affinities and competition of actin-binding toxins and profilin with MAL. A crystal structure of the MAL RPEL domain bound to three actin molecules revealed an additional actin-binding site within the RPEL 1-2 linker and actin-actin contacts in the RPEL domain-actin complex

Reconstitution of the destruction complex defines roles of AXIN polymers and APC in β-catenin capture, phosphorylation, and ubiquitylation.

The Wnt/β-catenin pathway is a highly conserved, frequently mutated developmental and cancer pathway. Its output is defined mainly by β-catenin's phosphorylation- and ubiquitylation-dependent proteasomal degradation, initiated by the multi-protein β-catenin destruction complex. The precise mechanisms underlying destruction complex function have remained unknown, largely because of the lack of suitable in vitro systems. Here we describe the in vitro reconstitution of an active human β-catenin destruction complex from purified components, recapitulating complex assembly, β-catenin modification, and degradation. We reveal that AXIN1 polymerization and APC promote β-catenin capture, phosphorylation, and ubiquitylation. APC facilitates β-catenin's flux through the complex by limiting ubiquitylation processivity and directly interacts with the SCFβ-TrCP E3 ligase complex in a β-TrCP-dependent manner. Oncogenic APC truncation variants, although part of the complex, are functionally impaired. Nonetheless, even the most severely truncated APC variant promotes β-catenin recruitment. These findings exemplify the power of biochemical reconstitution to interrogate the molecular mechanisms of Wnt/β-catenin signaling

Regulation of Wnt/β-catenin signalling by tankyrase-dependent poly(ADP-ribosyl)ation and scaffolding.

The Wnt/β-catenin signalling pathway is pivotal for stem cell function and the control of cellular differentiation, both during embryonic development and tissue homeostasis in adults. Its activity is carefully controlled through the concerted interactions of concentration-limited pathway components and a wide range of post-translational modifications, including phosphorylation, ubiquitylation, sumoylation, poly(ADP-ribosyl)ation (PARylation) and acetylation. Regulation of Wnt/β-catenin signalling by PARylation was discovered relatively recently. The PARP tankyrase PARylates AXIN1/2, an essential central scaffolding protein in the β-catenin destruction complex, and targets it for degradation, thereby fine-tuning the responsiveness of cells to the Wnt signal. The past few years have not only seen much progress in our understanding of the molecular mechanisms by which PARylation controls the pathway but also witnessed the successful development of tankyrase inhibitors as tool compounds and promising agents for the therapy of Wnt-dependent dysfunctions, including colorectal cancer. Recent work has hinted at more complex roles of tankyrase in Wnt/β-catenin signalling as well as challenges and opportunities in the development of tankyrase inhibitors. Here we review some of the latest advances in our understanding of tankyrase function in the pathway and efforts to modulate tankyrase activity to re-tune Wnt/β-catenin signalling in colorectal cancer cells.Linked articlesThis article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc

Fish in the City

Aquaculture is the most recent addition to animal husbandry and it is the fastest growing food production industry. Its contribution to world food security in the 21st century is already significant and it is bound to continue to grow because demand for fish for human consumption is rapidly increasing whereas fish supplies from ocean fisheries are likely to decline. The rapid evolution of aquaculture involved a host of innovations of which many were based on R&D activities by public and private research organizations. Applied R&D tends to be the more effective the better focused it is on specific research problems or opportunities. Among the many possible aquaculture production systems on which aquaculture R&D might focus are recirculation aquaculture systems and in this paper we explore crucial aspects of the potential of urban recirculation aquaculture. Our exploration begins with a vision of recirculation aquaculture production plants located at the fringes of cities of converging economies. Such production systems are distinctly different from conventional urban aquaculture systems based on urban sewage. We scrutinize our vision from four perspectives: (i) the expected demand for aquaculture fish from urban consumers; (ii) cost competitiveness of fish produced at the fringes of cities as compared to fish produced in the rural hinterland; (iii) the potential for integration of urban recirculation aquaculture production into the modern food supply chains that are now emerging in converging economies, and (iv) the ecological footprint of aquaculture production compared to that of chicken production. Based on trends in the growth of urban populations world-wide and trends in demand for fish for food we estimate a total urban demand for aquaculture finfish between 11 and 51 million tons in 2025. We use von Thünen's location theory to provide support for the vision to locate recirculation aquaculture plants not within cities and not in their rural hinterland but on the fringes of cities. Moreover, we argue that tightly controlled recirculation aquaculture production would seem to be particularly well suited for being integrated into modern food supply chains. Finally, we compare the ecological footprint of recirculation aquaculture fish with that of industrially produced chicken and we find that the ecological balance depends on the source of energy used. We conclude our exploratory study with some thoughts on the implication for aquaculture R&D of the potential for recirculation aquaculture located on the fringes of cities in emerging economy countries

Fragment-based screening identifies molecules targeting the substrate-binding ankyrin repeat domains of tankyrase.

The PARP enzyme and scaffolding protein tankyrase (TNKS, TNKS2) uses its ankyrin repeat clusters (ARCs) to bind a wide range of proteins and thereby controls diverse cellular functions. A number of these are implicated in cancer-relevant processes, including Wnt/β-catenin signalling, Hippo signalling and telomere maintenance. The ARCs recognise a conserved tankyrase-binding peptide motif (TBM). All currently available tankyrase inhibitors target the catalytic domain and inhibit tankyrase's poly(ADP-ribosyl)ation function. However, there is emerging evidence that catalysis-independent "scaffolding" mechanisms contribute to tankyrase function. Here we report a fragment-based screening programme against tankyrase ARC domains, using a combination of biophysical assays, including differential scanning fluorimetry (DSF) and nuclear magnetic resonance (NMR) spectroscopy. We identify fragment molecules that will serve as starting points for the development of tankyrase substrate binding antagonists. Such compounds will enable probing the scaffolding functions of tankyrase, and may, in the future, provide potential alternative therapeutic approaches to inhibiting tankyrase activity in cancer and other conditions

Solution NMR assignment of the ARC4 domain of human tankyrase 2.

Tankyrases are poly(ADP-ribose)polymerases (PARPs) which recognize their substrates via their ankyrin repeat cluster (ARC) domains. The human tankyrases (TNKS/TNKS2) contain five ARCs in their extensive N-terminal region; of these, four bind peptides present within tankyrase interactors and substrates. These short, linear segments, known as tankyrase-binding motifs (TBMs), contain some highly conserved features: an arginine at position 1, which occupies a predominantly acidic binding site, and a glycine at position 6 that is sandwiched between two aromatic side chains on the surface of the ARC domain. Tankyrases are involved in a multitude of biological functions, amongst them Wnt/β-catenin signaling, the maintenance of telomeres, glucose metabolism, spindle formation, the DNA damage response and Hippo signaling. As many of these are relevant to human disease, tankyrase is an important target candidate for drug development. With the emergence of non-catalytic (scaffolding) functions of tankyrase, it seems attractive to interfere with ARC function rather than the enzymatic activity of tankyrase. To study the mechanism of ARC-dependent recruitment of tankyrase binders and enable protein-observed NMR screening methods, we have as the first step obtained a full backbone and partial side chain assignment of TNKS2 ARC4. The assignment highlights some of the unusual structural features of the ARC domain

Electron and Hole Spin Splitting and Photogalvanic Effect in Quantum Wells

A theory of the circular photogalvanic effect caused by spin splitting in quantum wells is developed. Direct interband transitions between the hole and electron size-quantized subbands are considered. It is shown that the photocurrent value and direction depend strongly on the form of the spin-orbit interaction. The currents induced by structure-, bulk-, and interface-inversion asymmetry are investigated. The photocurrent excitation spectra caused by spin splittings in both conduction and valence bands are calculated.Comment: 7 pages, 3 figure

Regulation of Protein Interactions by <i>M</i>ps <i>O</i>ne <i>B</i>inder (MOB1) Phosphorylation.

MOB1 is a multifunctional protein best characterized for its integrative role in regulating Hippo and NDR pathway signaling in metazoans and the Mitotic Exit Network in yeast. Human MOB1 binds both the upstream kinases MST1 and MST2 and the downstream AGC group kinases LATS1, LATS2, NDR1, and NDR2. Binding of MOB1 to MST1 and MST2 is mediated by its phosphopeptide-binding infrastructure, the specificity of which matches the phosphorylation consensus of MST1 and MST2. On the other hand, binding of MOB1 to the LATS and NDR kinases is mediated by a distinct interaction surface on MOB1. By assembling both upstream and downstream kinases into a single complex, MOB1 facilitates the activation of the latter by the former through a trans-phosphorylation event. Binding of MOB1 to its upstream partners also renders MOB1 a substrate, which serves to differentially regulate its two protein interaction activities (at least in vitro). Our previous interaction proteomics analysis revealed that beyond associating with MST1 (and MST2), MOB1A and MOB1B can associate in a phosphorylation-dependent manner with at least two other signaling complexes, one containing the Rho guanine exchange factors (DOCK6-8) and the other containing the serine/threonine phosphatase PP6. Whether these complexes are recruited through the same mode of interaction as MST1 and MST2 remains unknown. Here, through a comprehensive set of biochemical, biophysical, mutational and structural studies, we quantitatively assess how phosphorylation of MOB1A regulates its interaction with both MST kinases and LATS/NDR family kinases in vitro Using interaction proteomics, we validate the significance of our in vitro studies and also discover that the phosphorylation-dependent recruitment of PP6 phosphatase and Rho guanine exchange factor protein complexes differ in key respects from that elucidated for MST1 and MST2. Together our studies confirm and extend previous work to delineate the intricate regulatory steps in key signaling pathways