A Novel, Evolutionarily Conserved Protein Phosphatase Complex Involved in Cisplatin Sensitivity

GammaH2AX and its role in DNA double-strand break repair

One of the earliest responses to a DNA double-strand break (DSB) is the carboxy-terminal phosphorylation of budding yeast H2A (metazoan histone H2AX) to create gammaH2A (or gammaH2AX). This chromatin modification stretches more than tens of kilobases around the DSB and has been proposed to play numerous roles in break recognition and repair, although it may not be the primary signal for many of these events. Studies suggest that gammaH2A(X) has 2 more direct roles: (i) to recruit cohesin around the DSB, and (ii) to maintain a checkpoint arrest. Recent work has identified other factors, including chromatin remodelers and protein phosphatases, which target gammaH2A(X) and regulate DSB repair/recovery.

Conditional knockout mice reveal an essential role of protein phosphatase 4 in thymocyte development and pre-T-cell receptor signaling

Okadaic acid-sensitive serine/threonine phosphatases have been shown to regulate interleukin-2 transcription and T-cell activation. Okadaic acid inhibits protein phosphatase 4 (PP4), a novel PP2A-related serine/threonine phosphatase, at a 50% inhibitory concentration (IC(50)) comparable to that for PP2A. This raises the possibility that some cellular functions of PP2A, determined in T cells by using okadaic acid, may in fact be those of PP4. To investigate the in vivo roles of PP4 in T cells, we generated conventional and T-cell-specific PP4 conditional knockout mice. We found that the ablation of PP4 led to the embryonic lethality of mice. PP4 gene deletion in the T-cell lineage resulted in aberrant thymocyte development, including T-cell arrest at the double-negative 3 stage (CD4(-) CD8(-) CD25(+) CD44(-)), abnormal thymocyte maturation, and lower efficacy of positive selection. PP4-deficient thymocytes showed decreased proliferation and enhanced apoptosis in vivo. Analysis of pre-T-cell receptor (pre-TCR) signaling further revealed impaired calcium flux and phospholipase C-gamma1-extracellular signal-regulated kinase activation in the absence of PP4. Anti-CD3 injection in PP4-deficient mice led to enhanced thymocyte apoptosis, accompanied by increased proapoptotic Bim but decreased antiapoptotic Bcl-xL protein levels. In the periphery, antigen-specific T-cell proliferation and T-cell-mediated immune responses in PP4-deficient mice were dramatically compromised. Thus, our results indicate that PP4 is essential for thymocyte development and pre-TCR signaling.

Identification and isolation of Dictyostelium microtubule-associated protein interactors by tandem affinity purification

Tandem affinity purification (TAP) is a method originally established in yeast to isolate highly purified protein complexes in a very gentle and efficient way. In this work, we have modified TAP for Dictyostelium applications and have proved it as a useful method to specifically isolate and identify microtubule-associated protein (MAP) complexes. MAPs are known to interact with other proteins to fulfill their complex functions in balancing the dynamic instability of microtubules as well as anchoring microtubules at the cell cortex, controlling mitosis at the centrosome and guiding transport along them. DdEB1 and the Dictyostelium member of the XMAP215 protein family, DdCP224, are known to be part of complexes at the microtubule tips as well as at the centrosome. Employing TAP and mass spectrometry we were able to prove an interaction between EB1 and the DdCP224. Additionally, among other interactions that remain to be confirmed by other methods, an interaction between DdCP224 and a TACC-family protein could be shown for the first time in Dictyostelium and was confirmed by colocalization and co-immunoprecipitation analyses.

SMK-1, an essential regulator of DAF-16-mediated longevity

Insulin/IGF-1 signaling (IIS) regulates aging in worms, flies, and mice through a well-characterized, highly conserved core set of components. IIS also regulates early developmental decisions, the reproductive status of the animal, innate immunity, and stress-resistance functions. In C. elegans, the sole insulin/IGF-1 receptor, DAF-2, negatively regulates the FOXO transcription factor, DAF-16. We report here on a new component of the IIS longevity pathway, SMK-1, which specifically influences DAF-16-dependent regulation of the aging process in C. elegans by regulating the transcriptional specificity of DAF-16 activity. Localization analysis of DAF-16 places SMK-1 downstream of DAF-16's phosphorylation-dependent relocation to the nucleus. Physiological and transcription analyses indicate that smk-1 is required for the innate immune, UV, and oxidative stress but not the thermal stress functions of DAF-16. SMK-1 therefore plays a role in longevity by modulating DAF-16 transcriptional specificity without affecting other processes regulated by IIS.

Histone post-translational modifications and the response to DNA double-strand breaks

The packaging of DNA into chromatin creates a number of significant barriers to the detection of DNA lesions and their timely and accurate repair. Eukaryotic cells have evolved a number of enzymes that modulate chromatin structure and facilitate DNA repair. Recent research illustrates how nucleosome remodelling enzymes cooperate with both DNA-damage-inducible and constitutive histone modifications to promote many facets of the cellular response to DNA damage.

The KLHL12-Cullin-3 ubiquitin ligase negatively regulates the Wnt-beta-catenin pathway by targeting Dishevelled for degradation

Dishevelled is a conserved protein that interprets signals received by Frizzled receptors. Using a tandem-affinity purification strategy and mass spectrometry we have identified proteins associated with Dishevelled, including a Cullin-3 ubiquitin ligase complex containing the Broad Complex, Tramtrack and Bric à Brac (BTB) protein Kelch-like 12 (KLHL12). This E3 ubiquitin ligase complex is recruited to Dishevelled in a Wnt-dependent manner that promotes its poly-ubiquitination and degradation. Functional analyses demonstrate that regulation of Dishevelled by this ubiquitin ligase antagonizes the Wnt-beta-catenin pathway in cultured cells, as well as in Xenopus and zebrafish embryos. Considered with evidence that the distinct Cullin-1 based SCF(beta-TrCP)complex regulates beta-catenin stability, our data on the stability of Dishevelled demonstrates that two distinct ubiquitin ligase complexes regulate the Wnt-beta-catenin pathway.