A decade has passed since SUMO (small ubiquitin-related modifier) was discovered to be a reversible post-translational protein modifier. During this time many enzymes that participate in regulated SUMO-conjugation and -deconjugation pathways have been identified and characterized. In parallel, the search for SUMO substrates has produced a long list of targets, which appear to be involved in most cellular functions. Sumoylation is a highly dynamic process and its outcomes are extremely diverse, ranging from changes in localization to altered activity and, in some cases, stability of the modified protein. At first glance, these effects have nothing in common; however, it seems that they all result from changes in the molecular interactions of the sumoylated proteins.

Concepts in sumoylation: a decade on

SUMO: a history of modification

Genome Regulation by Polycomb and Trithorax Proteins

Acquisition of the ability to evade cellular suicide, or apoptosis, is one of the master switches that contributes to cellular transformation and, ultimately, to invasive cancer. Much has been learned about the molecular organization of apoptotic pathways and their regulators, but the identification and validation of translational targets for apoptosis-based cancer therapy has posed a great challenge. Survivin is an attractive candidate for cancer therapy, so what is its potential applicability in the clinic?

Validating survivin as a cancer therapeutic target.

Aurora is the name given to a family of highly conserved protein kinases with essential roles in
many aspects of cell division. Yeasts have a single Aurora kinase, whereas mammals have three:
Aurora A, B and C. During mitosis, Aurora kinases regulate the structure and function of the cytoskeleton and chromosomes and the interactions between these two at the kinetochore.
They also regulate signalling by the spindle-assembly checkpoint pathway and cytokinesis.
Perturbation of Aurora kinase expression or function might lead to cancer.

/pdf/the-cellular-geography-of-aurora-kinases-4uaczeeap7.pdf

The cellular geography of aurora kinases.

Aurora B regulates chromosome segregation and cytokinesis and is the first protein to be implicated as a regulator of bipolar attachment of spindle microtubules to kinetochores. Evidence from several systems suggests that Aurora B is physically associated with inner centromere protein (INCENP) in mitosis and has genetic interactions with Survivin. It is unclear whether the Aurora B and INCENP interaction is cell cycle regulated and if Survivin physically interacts in this complex. In this study, we cloned the Xenopus Survivin gene, examined its association with Aurora B and INCENP, and determined the effect of its binding on Aurora B kinase activity. We demonstrate that in the Xenopus early embryo, all of the detectable Survivin is in a complex with both Aurora B and INCENP throughout the cell cycle. Survivin and Aurora B bind different domains on INCENP. Aurora B activity is stimulated >10-fold in mitotic extracts; this activation is phosphatase sensitive, and the binding of Survivin is required for full Aurora B activity. We also find the hydrodynamic properties of the Aurora B/Survivin/INCENP complex are cell cycle regulated. Our data indicate that Aurora B kinase activity is regulated by both Survivin binding and cell cycle-dependent phosphorylation.

Aurora B Kinase Exists in a Complex with Survivin and INCENP and Its Kinase Activity Is Stimulated by Survivin Binding and Phosphorylation

Received 18 April 2005/Returned for modification 17 June 2005/Accepted 4 November 2005 TGIF (TG-interacting factor) represses transforming growth factor (TGF-)-activated gene expression and can repress transcription via a specific retinoid response element. Mutations in human TGIF are associated with holoprosencephaly, a severe defect of craniofacial development with both genetic and environmental causes. Both TGF- and retinoic acid signaling are implicated in craniofacial development. Here, we analyze the role of TGIF in regulating retinoid responsive gene expression. We demonstrate that TGIF interacts with the ligand binding domain of the RXR retinoid receptor and represses transcription from retinoid response elements. TGIF recruits the general corepressor, CtBP, to RXR, and this recruitment is required for full repression by TGIF. Interaction between TGIF and RXR is reduced by the addition of retinoic acid, consistent with a role for TGIF as an RXR transcriptional corepressor. We created a Tgif null mutation in mice and tested the sensitivity of mutant mice to increased levels of retinoic acid. Tgif mutant embryos are more sensitive to retinoic acid-induced teratogenesis, and retinoid target genes are expressed at a higher level in tissues from Tgif null mice. These results demonstrate an important role for TGIF as a transcriptional corepressor, which regulates developmental signaling by retinoic acid, and raises the possibility that TGIF may repress other RXR-dependent transcriptional responses.

TGIF Inhibits Retinoid Signaling

Pc2 is a polycomb protein, which has SUMO E3 activity for the corepressors CtBP and CtBP2. Here we demonstrate that, in vivo, Pc2 adapter function contributes to enhancement of CtBP sumoylation. Mutation of the CtBP binding site on Pc2 abolishes E3 activity toward CtBP. However, a carboxyl-terminal fragment of Pc2 that recruits both Ubc9 and CtBP lacks E3 activity. We identify a second domain, which, when coexpressed with the carboxyl-terminal adapter region, restores E3 function. In vitro, this domain has E3 activity in isolation, suggesting that it is a functional domain, and that adapter function is required to selectively corecruit E2 and substrate in vivo. These results demonstrate the presence of two domains in Pc2 that contribute to full in vivo E3 activity, and suggest that SUMO E3s are more than simple platforms to which E2 and substrate bind.

/pdf/multiple-activities-contribute-to-pc2-e3-function-4hmlpetajy.pdf

Multiple activities contribute to Pc2 E3 function

Functional analysis of mutations in TGIF associated with holoprosencephaly

Tgif1 and Tgif2 are transcriptional co-repressors that limit the response to TGFβ signaling and play a role in regulating retinoic-acid-mediated gene expression. Mutations in human TGIF1 are associated with holoprosencephaly, but it is unclear whether this is a result of deregulation of TGFβ/Nodal signaling, or of effects on other pathways. Surprisingly, mutation of Tgif1 in mice results in only relatively mild developmental phenotypes in most strain backgrounds. Here, we show that loss-of-function mutations in both Tgif1 and Tgif2 result in a failure of gastrulation. By conditionally deleting Tgif1 in the epiblast, we demonstrate that a single wild-type allele of Tgif1 in the extra-embryonic tissue allows the double null embryos to gastrulate and begin organogenesis, suggesting that extra-embryonic Tgif function is required for patterning the epiblast. Genetically reducing the dose of Nodal in embryos lacking all Tgif function results in partial rescue of the gastrulation defects. Conditional double null embryos have defects in left-right asymmetry, which are also alleviated by reducing the dose of Nodal. Together, these data show that Tgif function is required for gastrulation, and provide the first clear evidence that Tgifs limit the transcriptional response to Nodal signaling during early embryogenesis.

/pdf/tgif1-and-tgif2-regulate-nodal-signaling-and-are-required-lp8iybah8y.pdf

Shannon E. Powers

Papers

Aurora B Kinase Exists in a Complex with Survivin and INCENP and Its Kinase Activity Is Stimulated by Survivin Binding and Phosphorylation

TGIF Inhibits Retinoid Signaling

Multiple activities contribute to Pc2 E3 function

Functional analysis of mutations in TGIF associated with holoprosencephaly

Tgif1 and Tgif2 regulate Nodal signaling and are required for gastrulation.