SummaryThe poly(ADP-ribose) polymerase (PARP) Tankyrase (TNKS and TNKS2) is paramount to Wnt-β-catenin signaling and a promising therapeutic target in Wnt-dependent cancers. The pool of active β-catenin is normally limited by destruction complexes, whose assembly depends on the polymeric master scaffolding protein AXIN. Tankyrase, which poly(ADP-ribosyl)ates and thereby destabilizes AXIN, also can polymerize, but the relevance of these polymers has remained unclear. We report crystal structures of the polymerizing TNKS and TNKS2 sterile alpha motif (SAM) domains, revealing versatile head-to-tail interactions. Biochemical studies informed by these structures demonstrate that polymerization is required for Tankyrase to drive β-catenin-dependent transcription. We show that the polymeric state supports PARP activity and allows Tankyrase to effectively access destruction complexes through enabling avidity-dependent AXIN binding. This study provides an example for regulated signal transduction in non-membrane-enclosed compartments (signalosomes), and it points to novel potential strategies to inhibit Tankyrase function in oncogenic Wnt signaling.

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

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Job (dis)satisfaction is related with psychological mechanisms in stress affecting cellular immune function.

The serine/threonine phosphatase protein phosphatase 5 (PP5) regulates hormone-and stress-induced cellular signaling by association with the molecular chaperone heat shock protein 90 (Hsp90). PP5-mediated dephosphorylation of the cochaperone Cdc37 is essential for activation of Hsp90-dependent kinases. However, the details of this mechanism remain unknown. We determined the crystal structure of a Cdc37 phosphomimetic peptide bound to the catalytic domain of PP5. The structure reveals PP5 utilization of conserved elements of phosphoprotein phosphatase (PPP) structure to bind substrate and provides a template for many PPP-substrate interactions. Our data show that, despite a highly conserved structure, elements of substrate specificity are determined within the phosphatase catalytic domain itself. Structure-based mutations in vivo reveal that PP5-mediated dephosphorylation is required for kinase and steroid hormone receptor release from the chaperone complex. Finally, our data show that hyper-or hypoactivity of PP5 mutants increases Hsp90 binding to its inhibitor, suggesting a mechanism to enhance the efficacy of Hsp90 inhibitors by regulation of PP5 activity in tumors.Hsp90 | PP5 | Cdc37 | chaperone | phosphatase P rotein phosphatase 5 (PP5) has pleiotropic roles in cellular signaling, including DNA damage repair, proliferation of breast cancer cells, circadian cycling, response to cytotoxic stresses, Rac-dependent potassium ion channel activity, and activation of steroid hormone receptors [e.g., glucocorticoid receptor (GR) and estrogen receptor] (1, 2). It is a member of the phosphoprotein phosphatase (PPP) family of serine/threonine phosphatases, which has members that share a highly conserved catalytic core and catalytic mechanism dependent on two metal ions, commonly Mn 2+ . Most PPP family members exhibit high, nonspecific phosphatase activity. Specificity is provided by a large cohort of regulatory and other interacting proteins that function to inhibit basal activity and recruit substrates, thereby finely tuning the enzymes (3). This combinatorial approach enables a small number of catalytic subunits to have the breadth of specificity equivalent to that seen in kinases, which are greater in number by an order of magnitude. Structures of complexes between regulatory and catalytic domains have illuminated the importance of regulatory subunits in facilitating substrate recruitment (3). However, to date, there is no structural information describing how a substrate binds at the active site of a PPP; therefore, a central question remains of how local interactions between the substrate and the catalytic domain contribute to the molecular basis of dephosphorylation.PP5 is unique among the PPP family because it has a low basal activity caused by an autoinhibitory N-terminal tetratricopeptide (TPR) domain (4). Its activity is promoted by a number of cellular factors, including fatty acids and the molecular chaperone heat shock protein 90 (Hsp90) (5), both of which release autoinhibition by interacting with the TPR...