hZwint-1 bridges the inner and outer kinetochore: identification of the kinetochore localization domain and the hZw10-interaction domain

Accurate chromosome segregation in mitosis is required to maintain genetic stability. hZwint-1 [human Zw10 (Zeste white 10)-interacting protein 1] is a kinetochore protein known to interact with the kinetochore checkpoint protein hZw10. hZw10, along with its partners Rod (Roughdeal) and hZwilch, form a complex which recruits dynein-dynactin and Mad1-Mad2 complexes to the kinetochore and are essential components of the mitotic checkpoint. hZwint-1 localizes to the kinetochore in prophase, before hZw10 localization, and remains at the kinetochore until anaphase, after hZw10 has dissociated. This difference in localization timing may reflect a role for hZwint-1 as a structural kinetochore protein. In addition to hZw10, we have found that hZwint-1 interacts with components of the conserved Ndc80 and Mis12 complexes in yeast two-hybrid and GST (glutathione transferase) pull-down assays. Furthermore, hZwint-1 was found to have stable FRAP (fluorescence recovery after photobleaching) dynamics similar to hHec1, hSpc24 and hMis12. As such, we proposed that hZwint-1 is a structural protein, part of the inner kinetochore scaffold and recruits hZw10 to the kinetochore. To test this, we performed mutagenesis-based domain mapping to determine which regions of hZwint-1 are necessary for kinetochore localization and which are required for interaction with hZw10. hZwint-1 localizes to the kinetochore through the N-terminal region and interacts with hZw10 through the C-terminal coiled-coil domain. The two domains are at opposite ends of the protein as expected for a protein that bridges the inner and outer kinetochore.     

Protein kinase A activity at the endoplasmic reticulum surface is responsible for augmentation of human ether-a-go-go-related gene product (HERG)

Human ether-a-go-go-related gene product (HERG) is a cardiac potassium channel commonly implicated in the pathogenesis of the long QT syndrome, type 2 (LQT2). LQT2 mutations typically have incomplete penetrance and affect individuals at various stages of their lives; this may mirror variations in intracellular signaling and HERG regulation. Previous work showed that sustained protein kinase A (PKA) activity augments HERG protein abundance by a mechanism that includes enhanced protein translation. To investigate the subcellular site of this regulation, we generated site-specific probes to the cytoplasmic surface of the endoplasmic reticulum (ER), the presumed locale of channel synthesis. Real-time FRET-based indicators demonstrated both cAMP and PKA activity at the ER. A PKA inhibitor targeted to the ER surface (termed p4PKIg) completely abolished PKA-mediated augmentation of HERG in HEK293 cells as well as rat neonatal cardiomyocytes. Immunofluorescence co-localization, targeted FRET-based PKA biosensors, phospho-specific antibodies, and in vivo phosphorylation experiments confirmed that p4PKIg is preferentially active at the ER surface rather than the plasma membrane. Rerouting this inhibitor to the outer mitochondrial membrane diminishes its ability to block cAMP-dependent HERG induction. Our results support a model where PKA-dependent regulation of HERG synthesis occurs at the ER surface. Furthermore, reagents generated for this study provide novel experimental tools to probe compartmentalized cAMP/PKA signaling within cells.     

Structure analysis of the Staphylococcus aureus UDP-N-acetyl-mannosamine dehydrogenase Cap5O involved in capsular polysaccharide biosynthesis

Bacterial UDP-sugar dehydrogenases are part of the biosynthesis pathway of extracellular polysaccharides. These compounds act as important virulence factors by protecting the cell from opsonophagocytosis and complement-mediated killing. In Staphylococcus aureus, the protein Cap5O catalyzes the oxidation of UDP-N-acetyl-mannosamine to UDP-N-acetyl-mannosaminuronic acid. Cap5O is crucial for the production of serotype 5 capsular polysaccharide that prevents the interaction of bacteria with both phagocytic and nonphagocytic eukaryotic cells. However, details of its catalytic mechanism remain unknown. We thus crystallized Cap5O and solved the first structure of an UDP-N-acetyl-mannosamine dehydrogenase. This study revealed that the catalytic cysteine makes a disulfide bond that has never been observed in other structurally characterized members of the NDP-sugar dehydrogenase family. Biochemical and mutagenesis experiments demonstrated that the formation of this disulfide bridge regulates the activity of Cap5O. We also identified two arginine residues essential for Cap5O activity. Previous data suggested that Cap5O is activated by tyrosine phosphorylation, so we characterized the phosphorylation site and examined the underlying regulatory mechanism.     

Primary hepatocytes from mice lacking cysteine dioxygenase show increased cysteine concentrations and higher rates of metabolism of cysteine to hydrogen sulfide and thiosulfate

The oxidation of cysteine in mammalian cells occurs by two routes: a highly regulated direct oxidation pathway in which the first step is catalyzed by cysteine dioxygenase (CDO) and by desulfhydration-oxidation pathways in which the sulfur is released in a reduced oxidation state. To assess the effect of a lack of CDO on production of hydrogen sulfide (H₂S) and thiosulfate (an intermediate in the oxidation of H₂S to sulfate) and to explore the roles of both cystathionine γ-lyase (CTH) and cystathionine β-synthase (CBS) in cysteine desulfhydration by liver, we investigated the metabolism of cysteine in hepatocytes isolated from Cdo1-null and wild-type mice. Hepatocytes from Cdo1-null mice produced more H₂S and thiosulfate than did hepatocytes from wild-type mice. The greater flux of cysteine through the cysteine desulfhydration reactions catalyzed by CTH and CBS in hepatocytes from Cdo1-null mice appeared to be the consequence of their higher cysteine levels, which were due to the lack of CDO and hence lack of catabolism of cysteine by the cysteinesulfinate-dependent pathways. Both CBS and CTH appeared to contribute substantially to cysteine desulfhydration, with estimates of 56 % by CBS and 44 % by CTH in hepatocytes from wild-type mice, and 63 % by CBS and 37 % by CTH in hepatocytes from Cdo1-null mice.     

Crystal structure of the catalytic core of Rad2: insights into the mechanism of substrate binding

Rad2/XPG belongs to the flap nuclease family and is responsible for a key step of the eukaryotic nucleotide excision DNA repair (NER) pathway. To elucidate the mechanism of DNA binding by Rad2/XPG, we solved crystal structures of the catalytic core of Rad2 in complex with a substrate. Rad2 utilizes three structural modules for recognition of the double-stranded portion of DNA substrate, particularly a Rad2-specific α-helix for binding the cleaved strand. The protein does not specifically recognize the single-stranded portion of the nucleic acid. Our data suggest that in contrast to related enzymes (FEN1 and EXO1), the Rad2 active site may be more accessible, which would create an exit route for substrates without a free 5′ end.     

Histone H3 lysine 27 acetylation is altered in colon cancer

Background

Histone post-translational modifications (PTMs) play an important role in the regulation of the expression of genes, including those involved in cancer development and progression. However, our knowledge of PTM patterns in human tumours is limited.

Methods

MS-based analyses were used to quantify global alterations of histone PTMs in colorectal cancer (CRC) samples. Histones isolated from 12 CRCs and their corresponding normal mucosa by acidic extraction were separated by SDS-PAGE and analysed by liquid chromatography-mass spectrometry.

Results

Among 96 modified peptides, 41 distinct PTM sites were identified, of which 7, 13, 11, and 10 were located within the H2A, H2B, H3, and H4 sequences, respectively, and distributed among the amino-terminal tails and the globular domain of the four histones. Modification intensities were quantified for 33 sites, of which 4 showed significant (p-value ≤ 0.05) differences between CRC tissues and healthy mucosa samples. We identified histone H3 lysine 27 acetylation (H3K27Ac) as a modification upregulated in CRC, which had not been shown previously.

Conclusions

The present results indicate the usefulness of a bottom-up proteomic approach for the detection of histone modifications at a global scale. The differential abundance of H3K27Ac mark in CRC, a PTM associated with active enhancers, suggests its role in regulating genes whose expression changes in CRC.     

Synaptically Released Matrix Metalloproteinase Activity in Control of Structural Plasticity and the Cell Surface Distribution of GluA1-AMPA Receptors

Synapses are particularly prone to dynamic alterations and thus play a major role in neuronal plasticity. Dynamic excitatory synapses are located at the membranous neuronal protrusions called dendritic spines. The ability to change synaptic connections involves both alterations at the morphological level and changes in postsynaptic receptor composition. We report that endogenous matrix metalloproteinase (MMP) activity promotes the structural and functional plasticity of local synapses by its effect on glutamate receptor mobility and content. We used live imaging of cultured hippocampal neurons and quantitative morphological analysis to show that chemical long-term potentiation (cLTP) induces the permanent enlargement of a subset of small dendritic spines in an MMP-dependent manner. We also used a superresolution microscopy approach and found that spine expansion induced by cLTP was accompanied by MMP-dependent immobilization and synaptic accumulation as well as the clustering of GluA1-containing AMPA receptors. Altogether, our results reveal novel molecular and cellular mechanisms of synaptic plasticity.