Project description:Promyelocytic leukemia protein (PML) is the core component of PML-nuclear bodies (PML NBs). The small ubiquitin-like modifier (SUMO) system (and, in particular, SUMOylation of PML) is a critical component in the formation and regulation of PML NBs. SUMO protease SENP6 has been shown previously to be specific for SUMO-2/3-modified substrates and shows preference for SUMO polymers. Here, we further investigate the substrate specificity of SENP6 and show that it is also capable of cleaving mixed chains of SUMO-1 and SUMO-2/3. Depletion of SENP6 results in accumulation of endogenous SUMO-2/3 and SUMO-1 conjugates, and immunofluorescence analysis shows accumulation of SUMO and PML in an increased number of PML NBs. Although SENP6 depletion drastically increases the size of PML NBs, the organizational structure of the body is not affected. Mutation of the catalytic cysteine of SENP6 results in its accumulation in PML NBs, and biochemical analysis indicates that SUMO-modified PML is a substrate of SENP6.
Project description:Outer kinetochore assembly enables chromosome attachment to microtubules and spindle assembly checkpoint (SAC) signaling in mitosis. Aurora B kinase controls kinetochore assembly by phosphorylating the Mis12 complex (Mis12C) subunit Dsn1. Current models propose Dsn1 phosphorylation relieves autoinhibition, allowing Mis12C binding to inner kinetochore component CENP-C. Using Xenopus laevis egg extracts and biochemical reconstitution, we found that autoinhibition of the Mis12C by Dsn1 impedes its phosphorylation by Aurora B. Our data indicate that the INCENP central region increases Dsn1 phosphorylation by enriching Aurora B at inner kinetochores, close to CENP-C. Furthermore, centromere-bound CENP-C does not exchange in mitosis, and CENP-C binding to the Mis12C dramatically increases Dsn1 phosphorylation by Aurora B. We propose that the coincidence of Aurora B and CENP-C at inner kinetochores ensures the fidelity of kinetochore assembly. We also found that the central region is required for the SAC beyond its role in kinetochore assembly, suggesting that kinetochore enrichment of Aurora B promotes the phosphorylation of other kinetochore substrates.
Project description:SUMO proteases can regulate the amounts of SUMO-conjugated proteins in the cell by cleaving off the isopeptidic bond between SUMO and the target protein. Of the six members that constitute the human SENP/ULP protease family, SENP6 and SENP7 are the most divergent members in their conserved catalytic domain. The SENP6 and SENP7 subclass displays a clear proteolytic cleavage preference for SUMO2/3 isoforms. To investigate the structural determinants for such isoform specificity, we have identified a unique sequence insertion in the SENP6 and SENP7 subclass that is essential for their proteolytic activity and that forms a more extensive interface with SUMO during the proteolytic reaction. Furthermore, we have identified a region in the SUMO surface determinant for the SUMO2/3 isoform specificity of SENP6 and SENP7. Double point amino acid mutagenesis on the SUMO surface allows us to swap the specificity of SENP6 and SENP7 between the two SUMO isoforms. Structure-based comparisons combined with biochemical and mutagenesis analysis have revealed Loop 1 insertion in SENP6 and SENP7 as a platform to discriminate between SUMO1 and SUMO2/3 isoforms in this subclass of the SUMO protease family.
Project description:To investigate the dynamics of centromere organization, we have assessed the exchange rates of inner centromere proteins (CENPs) by quantitative microscopy throughout the cell cycle in human cells. CENP-A and CENP-I are stable centromere components that are incorporated into centromeres via a "loading-only" mechanism in G1 and S phase, respectively. A subfraction of CENP-H also stays stably bound to centromeres. In contrast, CENP-B, CENP-C, and some CENP-H and hMis12 exhibit distinct and cell cycle-specific centromere binding stabilities, with residence times ranging from seconds to hours. CENP-C and CENP-H are immobilized at centromeres specifically during replication. In mitosis, all inner CENPs become completely immobilized. CENPs are highly mobile throughout bulk chromatin, which is consistent with a binding-diffusion behavior as the mechanism to scan for vacant high-affinity binding sites at centromeres. Our data reveal a wide range of cell cycle-specific assembly plasticity of the centromere that provides both stability through sustained binding of some components and flexibility through dynamic exchange of other components.
Project description:SUMO (small ubiquitin-related modifier) is a member of the ubiquitin-like protein family that regulates cellular function of a variety of target proteins. SUMO proteins are expressed as their precursor forms. Cleavage of the residues after the 'GG' region of these precursors by SUMO-specific proteases in maturation is a prerequisite for subsequent sumoylation. To understand further this proteolytic processing, we expressed and purified SENP1 (sentrin-specific protease 1), one of the SUMO-specific proteases, using an Escherichia coli expression system. We show that SENP1 is capable of processing all SUMO-1, -2 and -3 in vitro; however, the proteolytic efficiency of SUMO-1 is the highest followed by SUMO-2 and -3. We demonstrate further that the catalytic domain of SENP1 (SENP1C) alone can determine the substrate specificity towards SUMO-1, -2 and -3. Replacement of the C-terminal fragments after the 'GG' region of SUMO-1 and -2 precursors with that of the SUMO-3, indicates that the C-terminal fragment is essential for efficient maturation. In mutagenesis analysis, we further map two residues immediately after the 'GG' region, which determine the differential maturation. Distinct patterns of tissue distribution of SENP1, SUMO-1, -2 and -3 are characterized. Taken together, we suggest that the observed differential maturation process has its physiological significance in the regulation of the sumoylation pathway.
Project description:Kinetochores link centromeric DNA to spindle microtubules and ensure faithful chromosome segregation during mitosis. In point-centromere yeasts, the CBF3 complex Skp1-Ctf13-(Cep3)(2)-(Ndc10)(2) recognizes a conserved centromeric DNA element through contacts made by Cep3 and Ndc10. We describe here the five-domain organization of Kluyveromyces lactis Ndc10 and the structure at 2.8 Å resolution of domains I-II (residues 1-402) bound to DNA. The structure resembles tyrosine DNA recombinases, although it lacks both endonuclease and ligase activities. Structural and biochemical data demonstrate that each subunit of the Ndc10 dimer binds a separate fragment of DNA, suggesting that Ndc10 stabilizes a DNA loop at the centromere. We describe in vitro association experiments showing that specific domains of Ndc10 interact with each of the known inner-kinetochore proteins or protein complexes in budding yeast. We propose that Ndc10 provides a central platform for inner-kinetochore assembly.
Project description:In contrast to our extensive knowledge on ubiquitin polymer signaling, we are severely limited in our understanding of poly-SUMO signaling. We set out to identify substrates conjugated to SUMO polymers, using knockdown of the poly-SUMO2/3 protease SENP6. We identify over 180 SENP6 regulated proteins that represent highly interconnected functional groups of proteins including the constitutive centromere-associated network (CCAN), the CENP-A loading factors Mis18BP1 and Mis18A and DNA damage response factors. Our results indicate a striking protein group de-modification by SENP6. SENP6 deficient cells are severely compromised for proliferation, accumulate in G2/M and frequently form micronuclei. Accumulation of CENP-T, CENP-W and CENP-A to centromeres is impaired in the absence of SENP6. Surprisingly, the increase of SUMO chains does not lead to ubiquitin-dependent proteasomal degradation of the CCAN subunits. Our results indicate that SUMO polymers can act in a proteolysis-independent manner and consequently, have a more diverse signaling function than previously expected.
Project description:The signaling of Toll-like receptors (TLRs) induces host defense against microbial invasion. Protein posttranslational modifications dynamically shape the strength and duration of the signaling pathways. It is intriguing to explore whether de-SUMOylation could modulate the TLR signaling. Here we identified SUMO-specific protease 6 (SENP6) as an intrinsic attenuator of the TLR-triggered inflammation. Depletion of SENP6 significantly potentiated the NF-?B-mediated induction of the proinflammatory genes. Consistently, SENP6-knockdown mice were more susceptible to endotoxin-induced sepsis. Mechanistically, the small ubiquitin-like modifier 2/3 (SUMO-2/3) is conjugated onto the Lysine residue 277 of NF-?B essential modifier (NEMO/IKK?), and this impairs the deubiquitinase CYLD to bind NEMO, thus strengthening the inhibitor of ?B kinase (IKK) activation. SENP6 reverses this process by catalyzing the de-SUMOylation of NEMO. Our study highlights the essential function of the SENP family in dampening TLR signaling and inflammation.
Project description:The SUMO protein is covalently attached to many different substrates throughout the cell. This modification is rapidly reversed by SUMO proteases. The Saccharomyces cerevisiae SUMO protease Ulp2 is a nuclear protein required for chromosome stability and cell cycle restart after checkpoint arrest. Ulp2 is related to the human SENP6 protease, also a nuclear protein. All members of the Ulp2/SENP6 family of SUMO proteases have large but poorly conserved N-terminal domains (NTDs) adjacent to the catalytic domain. Ulp2 also has a long C-terminal domain (CTD). We show that CTD deletion has modest effects on yeast growth, but poly-SUMO conjugates accumulate. In contrast, the NTD is essential for Ulp2 function and is required for nuclear targeting. Two short, widely separated sequences within the NTD confer nuclear localization. Efficient Ulp2 import into the nucleus requires the beta-importin Kap95, which functions on classical nuclear-localization signal (NLS)-bearing substrates. Remarkably, replacement of the entire >400-residue NTD by a heterologous NLS results in near-normal Ulp2 function. These data demonstrate that nuclear localization of Ulp2 is crucial in vivo, yet only small segments of the NTD provide the key functional elements, explaining the minimal sequence conservation of the NTDs in the Ulp2/SENP6 family of enzymes.
Project description:SUMOylation is a dynamic process, catalyzed by SUMO-specific ligases and reversed by Sentrin/SUMO-specific proteases (SENPs). The physiologic consequences of SUMOylation and deSUMOylation are not fully understood. Here we investigate the phenotypes of mice lacking SENP1 and find that SENP1(-/-) embryos show severe fetal anemia stemming from deficient erythropoietin (Epo) production and die midgestation. We determine that SENP1 controls Epo production by regulating the stability of hypoxia-inducible factor 1alpha (HIF1alpha) during hypoxia. Hypoxia induces SUMOylation of HIF1alpha, which promotes its binding to a ubiquitin ligase, von Hippel-Lindau (VHL) protein, through a proline hydroxylation-independent mechanism, leading to its ubiquitination and degradation. In SENP1(-/-) MEFs, hypoxia-induced transcription of HIF1alpha-dependent genes such as vascular endothelial growth factor (VEGF) and glucose transporter 1 (Glut-1) is markedly reduced. These results show that SENP1 plays a key role in the regulation of the hypoxic response through regulation of HIF1alpha stability and that SUMOylation can serve as a direct signal for ubiquitin-dependent degradation.