The TFIIH subunit Tfb3 regulates cullin neddylation.

Cullin proteins are scaffolds for the assembly of multisubunit ubiquitin ligases, which ubiquitylate a large number of proteins involved in widely varying cellular functions. Multiple mechanisms cooperate to regulate cullin activity, including neddylation of their C-terminal domain. Interestingly, we found that the yeast Cul4-type cullin Rtt101 is not only neddylated but also ubiquitylated, and both modifications promote Rtt101 function in vivo. Surprisingly, proper modification of Rtt101 neither correlated with catalytic activity of the RING domain of Hrt1 nor required the Nedd8 ligase Dcn1. Instead, ubiquitylation of Rtt101 was dependent on the ubiquitin-conjugating enzyme Ubc4, while efficient neddylation involves the RING domain protein Tfb3, a subunit of the transcription factor TFIIH. Tfb3 also controls Cul3 neddylation and activity in vivo, and physically interacts with Ubc4 and the Nedd8-conjugating enzyme Ubc12 and the Hrt1/Rtt101 complex. Together, these results suggest that the conserved RING domain protein Tfb3 controls activation of a subset of cullins.

Global map of SUMO function revealed by protein-protein interaction and genetic networks.

Systematic functional genomics approaches were used to map a network centered on the small ubiquitin-related modifier (SUMO) system. Over 250 physical interactions were identified using the SUMO protein as bait in affinity purification-mass spectrometry and yeast two-hybrid screens. More than 500 genes and 1400 synthetic genetic interactions were mapped by synthetic genetic array (SGA) analysis using eight different SUMO pathway query genes. The resultant global SUMO network highlights its role in 15 major biological processes and better defines functional relationships between the different components of the SUMO pathway. Using this information-rich resource, we have identified roles for the SUMO system in the function of the AAA ATPase Cdc48p, the regulation of lipid metabolism, localization of the ATP-dependent endonuclease Dna2p, and recovery from the DNA-damage checkpoint.

A negative genetic interaction map in isogenic cancer cell lines reveals cancer cell vulnerabilities.

Improved efforts are necessary to define the functional product of cancer mutations currently being revealed through large-scale sequencing efforts. Using genome-scale pooled shRNA screening technology, we mapped negative genetic interactions across a set of isogenic cancer cell lines and confirmed hundreds of these interactions in orthogonal co-culture competition assays to generate a high-confidence genetic interaction network of differentially essential or differential essentiality (DiE) genes. The network uncovered examples of conserved genetic interactions, densely connected functional modules derived from comparative genomics with model systems data, functions for uncharacterized genes in the human genome and targetable vulnerabilities. Finally, we demonstrate a general applicability of DiE gene signatures in determining genetic dependencies of other non-isogenic cancer cell lines. For example, the PTEN(-/-) DiE genes reveal a signature that can preferentially classify PTEN-dependent genotypes across a series of non-isogenic cell lines derived from the breast, pancreas and ovarian cancers. Our reference network suggests that many cancer vulnerabilities remain to be discovered through systematic derivation of a network of differentially essential genes in an isogenic cancer cell model.

Structure of minimal tetratricopeptide repeat domain protein Tah1 reveals mechanism of its interaction with Pih1 and Hsp90.

Tah1 and Pih1 are novel Hsp90 interactors. Tah1 acts as a cofactor of Hsp90 to stabilize Pih1. In yeast, Hsp90, Tah1, and Pih1 were found to form a complex that is required for ribosomal RNA processing through their effect on box C/D small nucleolar ribonucleoprotein assembly. Tah1 is a minimal tetratricopeptide repeat protein of 111 amino acid residues that binds to the C terminus of the Hsp90 molecular chaperone, whereas Pih1 consists of 344 residues of unknown fold. The NMR structure of Tah1 has been solved, and this structure shows the presence of two tetratricopeptide repeat motifs followed by a C helix and an unstructured region. The binding of Tah1 to Hsp90 is mediated by the EEVD C-terminal residues of Hsp90, which bind to a positively charged channel formed by Tah1. Five highly conserved residues, which form a two-carboxylate clamp that tightly interacts with the ultimate Asp-0 residue of the bound peptide, are also present in Tah1. Tah1 was found to bind to the C terminus of Pih1 through the C helix and the unstructured region. The C terminus of Pih1 destabilizes the protein in vitro and in vivo, whereas the binding of Tah1 to Pih1 allows for the formation of a stable complex. Based on our data, a model for an Hsp90-Tah1-Pih1 ternary complex is proposed.

The role of Hsp90 in protein complex assembly.

Hsp90 is a ubiquitous and essential molecular chaperone that plays central roles in many signaling and other cellular pathways. The in vivo and in vitro activity of Hsp90 depends on its association with a wide variety of cochaperones and cofactors, which form large multi-protein complexes involved in folding client proteins. Based on our proteomic work mapping the molecular chaperone interaction networks in yeast, especially that of Hsp90, as well as, on experiments and results presented in the published literature, one major role of Hsp90 appears to be the promotion and maintenance of proper assembly of protein complexes. To highlight this role of Hsp90, the effect of the chaperone on the assembly of the following seven complexes is discussed in this review: snoRNP, RNA polymerase II, phosphatidylinositol-3 kinase-related protein kinase (PIKK), telomere complex, kinetochore, RNA induced silencing complexes (RISC), and 26S proteasome. For some complexes, it is observed that Hsp90 mediates complex assembly by stabilizing an unstable protein subunit and facilitating its incorporation into the complex; for other complexes, Hsp90 promotes change in the composition of that complex. In all cases, Hsp90 does not appear to be part of the final assembled complex. This article is part of a Special Issue entitled:Heat Shock Protein 90 (HSP90).

The role of karyopherins in the regulated sumoylation of septins.

In the yeast Saccharomyces cerevisiae, several components of the septin ring are sumoylated during anaphase and then abruptly desumoylated at cytokinesis. We show that septin sumoylation is controlled by the interactions of two enzymes of the sumoylation pathway, Siz1p and Ulp1p, with the nuclear transport machinery. The E3 ligase Siz1p is imported into the nucleus by the karyopherin Kap95p during interphase. In M phase, Siz1p is exported from the nucleus by the karyopherin Kap142p/Msn5p and subsequently targeted to the septin ring, where it participates in septin sumoylation. We also show that the accumulation of sumoylated septins during mitosis is dependent on the interactions of the SUMO isopeptidase Ulp1p with Kap121p and Kap95p-Kap60p and the nuclear pore complex (NPC). In addition to sequestering Ulp1 at the NPC, Kap121p is required for targeting Ulp1p to the septin ring during mitosis. We present a model in which Ulp1p is maintained at the NPC during interphase and transiently interacts with the septin ring during mitosis.

Karyopherins in nuclear pore biogenesis: a role for Kap121p in the assembly of Nup53p into nuclear pore complexes.

The mechanisms that govern the assembly of nuclear pore complexes (NPCs) remain largely unknown. Here, we have established a role for karyopherins in this process. We show that the yeast karyopherin Kap121p functions in the targeting and assembly of the nucleoporin Nup53p into NPCs by recognizing a nuclear localization signal (NLS) in Nup53p. This karyopherin-mediated function can also be performed by the Kap95p-Kap60p complex if the Kap121p-binding domain of Nup53p is replaced by a classical NLS, suggesting a more general role for karyopherins in NPC assembly. At the NPC, neighboring nucleoporins bind to two regions in Nup53p. One nucleoporin, Nup170p, associates with a region of Nup53p that overlaps with the Kap121p binding site and we show that they compete for binding to Nup53p. We propose that once targeted to the NPC, dissociation of the Kap121p-Nup53p complex is driven by the interaction of Nup53p with Nup170p. At the NPC, Nup53p exists in two separate complexes, one of which is capable of interacting with Kap121p and another that is bound to Nup170p. We propose that fluctuations between these two states drive the binding and release of Kap121p from Nup53p, thus facilitating Kap121p's movement through the NPC.

Yeast Two-Hybrid Analysis for Ubiquitin Variant Inhibitors of Human Deubiquitinases.

We applied a yeast-two-hybrid (Y2H) analysis to screen for ubiquitin variant (UbV) inhibitors of a human deubiquitinase (DUB), ubiquitin-specific protease 2 (USP2). The Y2H screen used USP2 as the bait and a prey library consisting of UbVs randomized at four specific positions, which were known to interact with USP2 from phage display analysis. The screen yielded numerous UbVs that bound to USP2 both as a Y2H interaction in vivo and as purified proteins in vitro. The Y2H-derived UbVs inhibited the catalytic activity of USP2 in vitro with nanomolar-range potencies, and they bound and inhibited USP2 in human cells. Mutational and structural analysis showed that potent and selective inhibition could be achieved by just two substitutions in a UbV, which exhibited improved hydrophobic and hydrophilic contacts compared to the wild-type ubiquitin interaction with USP2. Our results establish Y2H as an effective platform for the development of UbV inhibitors of DUBs in vivo, providing an alternative strategy for the analysis of DUBs that are recalcitrant to phage display and other in vitro methods.

Generation and Validation of Intracellular Ubiquitin Variant Inhibitors for USP7 and USP10.

Post-translational modification of the p53 signaling pathway plays an important role in cell cycle progression and stress-induced apoptosis. Indeed, a large body of work has shown that dysregulation of p53 and its E3 ligase MDM2 by the ubiquitin-proteasome system (UPS) promotes carcinogenesis and malignant transformation. Thus, drug discovery efforts have focused on the restoration of wild-type p53 activity or inactivation of oncogenic mutant p53 by targeted inhibition of UPS components, particularly key deubiquitinases (DUBs) of the ubiquitin-specific protease (USP) class. However, development of selective small-molecule USP inhibitors has been challenging, partly due to the highly conserved structural features of the catalytic sites across the class. To tackle this problem, we devised a protein engineering strategy for rational design of inhibitors for DUBs and other UPS proteins. We employed a phage-displayed ubiquitin variant (UbV) library to develop inhibitors targeting the DUBs USP7 and USP10, which are involved in regulating levels of p53 and MDM2. We were able to identify UbVs that bound USP7 or USP10 with high affinity and inhibited deubiquitination activity. We solved the crystal structure of UbV.7.2 and rationalized the molecular basis for enhanced affinity and specificity for USP7. Finally, cell death was increased significantly by UbV.7.2 expression in a colon cancer cell line that was treated with the chemotherapy drug cisplatin, demonstrating the therapeutic potential of inhibiting USP7 by this approach.

Nop53p is a novel nucleolar 60S ribosomal subunit biogenesis protein.

Ribosome biogenesis in Saccharomyces cerevisiae occurs primarily in a specialized nuclear compartment termed the nucleolus within which the rRNA genes are transcribed by RNA polymerase I into a large 35 S rRNA precursor. The ensuing association/dissociation and catalytic activity of numerous trans-acting protein factors, RNAs and ribosomal proteins ultimately leads to the maturation of the precursor rRNAs into 25, 5.8 and 18 S rRNAs and the formation of mature cytoplasmic 40 and 60 S ribosomal subunits. Although many components involved in ribosome biogenesis have been identified, our understanding of this essential cellular process remains limited. In the present study we demonstrate a crucial role for the previously uncharacterized nucleolar protein Nop53p (Ypl146p) in ribosome biogenesis. Specifically, Nop53p appears to be most important for biogenesis of the 60 S subunit. It physically interacts with rRNA processing factors, notably Cbf5p and Nop2p, and co-fractionates specifically with pre-60 S particles on sucrose gradients. Deletion or mutations within NOP53 cause significant growth defects and display significant 60 S subunit deficiencies, an imbalance in the 40 S:60 S ratio, as revealed by polysome profiling, and defects in progression beyond the 27 S stage of 25 S rRNA maturation during 60 S biogenesis.

Nutritional status modulates box C/D snoRNP biogenesis by regulated subcellular relocalization of the R2TP complex.

BACKGROUND: Box C/D snoRNPs, which are typically composed of box C/D snoRNA and the four core protein components Nop1, Nop56, Nop58, and Snu13, play an essential role in the modification and processing of pre-ribosomal RNA. The highly conserved R2TP complex, comprising the proteins Rvb1, Rvb2, Tah1, and Pih1, has been shown to be required for box C/D snoRNP biogenesis and assembly; however, the molecular basis of R2TP chaperone-like activity is not yet known. RESULTS: Here, we describe an unexpected finding in which the activity of the R2TP complex is required for Nop58 protein stability and is controlled by the dynamic subcellular redistribution of the complex in response to growth conditions and nutrient availability. In growing cells, the complex localizes to the nucleus and interacts with box C/D snoRNPs. This interaction is significantly reduced in poorly growing cells as R2TP predominantly relocalizes to the cytoplasm. The R2TP-snoRNP interaction is mainly mediated by Pih1. CONCLUSIONS: The R2TP complex exerts a novel regulation on box C/D snoRNP biogenesis that affects their assembly and consequently pre-rRNA maturation in response to different growth conditions.

The control of spindle length by Hsp70 and Hsp110 molecular chaperones.

Molecular chaperones are an essential group of proteins required to maintain proper protein homeostasis in the cell and include Hsp40, Hsp60, Hsp70, Hsp90, and Hsp100 among others. Hsp110 proteins form a subfamily of the Hsp70 family and seem to primarily function as nucleotide exchange factors for the Hsp70s. Data to date suggest that Hsp110 together with Hsp70 are required to ensure proper spindle assembly and nuclear distribution during cell division. More specifically, we propose that an Hsp110-Hsp70 complex modulates the activity and directionality of the kinesin-5 motor, Cin8, which is required for spindle elongation. The modulation of spindle length by molecular chaperones might be a mechanism by which cell division can be controlled especially under proteostatic stress.

Hsp110 is required for spindle length control.

Systematic affinity purification combined with mass spectrometry analysis of N- and C-tagged cytoplasmic Hsp70/Hsp110 chaperones was used to identify new roles of Hsp70/Hsp110 in the cell. This allowed the mapping of a chaperone-protein network consisting of 1,227 unique interactions between the 9 chaperones and 473 proteins and highlighted roles for Hsp70/Hsp110 in 14 broad biological processes. Using this information, we uncovered an essential role for Hsp110 in spindle assembly and, more specifically, in modulating the activity of the widely conserved kinesin-5 motor Cin8. The role of Hsp110 Sse1 as a nucleotide exchange factor for the Hsp70 chaperones Ssa1/Ssa2 was found to be required for maintaining the proper distribution of kinesin-5 motors within the spindle, which was subsequently required for bipolar spindle assembly in S phase. These data suggest a model whereby the Hsp70-Hsp110 chaperone complex antagonizes Cin8 plus-end motility and prevents premature spindle elongation in S phase.

Lumenal interactions in nuclear pore complex assembly and stability.

Nuclear pore complexes (NPCs) provide a gateway for the selective transport of macromolecules across the nuclear envelope (NE). Although we have a solid understanding of NPC composition and structure, we do not have a clear grasp of the mechanism of NPC assembly. Here, we demonstrate specific defects in nucleoporin distribution in strains lacking Heh1p and Heh2p-two conserved members of the LEM (Lap2, emerin, MAN1) family of integral inner nuclear membrane proteins. These effects on nucleoporin localization are likely of functional importance as we have defined specific genetic interaction networks between HEH1 and HEH2, and genes encoding nucleoporins in the membrane, inner, and outer ring complexes of the NPC. Interestingly, expression of a domain of Heh1p that resides in the NE lumen is sufficient to suppress both the nucleoporin mislocalization and growth defects in heh1Δpom34Δ strains. We further demonstrate a specific physical interaction between the Heh1p lumenal domain and the massive cadherin-like lumenal domain of the membrane nucleoporin Pom152p. These findings support a role for Heh1p in the assembly or stability of the NPC, potentially through the formation of a lumenal bridge with Pom152p.

A novel mechanism for SUMO system control: regulated Ulp1 nucleolar sequestration.

The small ubiquitin-related modifiers (SUMOs) are evolutionarily conserved polypeptides that are covalently conjugated to protein targets to modulate their subcellular localization, half-life, or activity. Steady-state SUMO conjugation levels increase in response to many different types of environmental stresses, but how the SUMO system is regulated in response to these insults is not well understood. Here, we characterize a novel mode of SUMO system control: in response to elevated alcohol levels, the Saccharomyces cerevisiae SUMO protease Ulp1 is disengaged from its usual location at the nuclear pore complex (NPC) and sequestered in the nucleolus. We further show that the Ulp1 region previously demonstrated to interact with the karyopherins Kap95 and Kap60 (amino acids 150 to 340) is necessary and sufficient for nucleolar targeting and that enforced sequestration of Ulp1 in the nucleolus significantly increases steady-state SUMO conjugate levels, even in the absence of alcohol. We have thus characterized a novel mechanism of SUMO system control in which the balance between SUMO-conjugating and -deconjugating activities at the NPC is altered in response to stress via relocalization of a SUMO-deconjugating enzyme.

The genetic landscape of a cell.

A genome-scale genetic interaction map was constructed by examining 5.4 million gene-gene pairs for synthetic genetic interactions, generating quantitative genetic interaction profiles for approximately 75% of all genes in the budding yeast, Saccharomyces cerevisiae. A network based on genetic interaction profiles reveals a functional map of the cell in which genes of similar biological processes cluster together in coherent subsets, and highly correlated profiles delineate specific pathways to define gene function. The global network identifies functional cross-connections between all bioprocesses, mapping a cellular wiring diagram of pleiotropy. Genetic interaction degree correlated with a number of different gene attributes, which may be informative about genetic network hubs in other organisms. We also demonstrate that extensive and unbiased mapping of the genetic landscape provides a key for interpretation of chemical-genetic interactions and drug target identification.

Nup53p is a target of two mitotic kinases, Cdk1p and Hrr25p.

Nuclear pore complexes (NPCs) form channels across the nuclear envelope and provide the sole sites of molecular exchange between the cytoplasm and nucleoplasm. The NPC is a target of a number of post-translational modifications, including phosphorylation, yet the functions of these modifications are ill defined. Here, we have investigated the mitotic specific phosphorylation of a yeast nucleoporin Nup53p. Two kinases were identified that phosphorylate Nup53p: the mitotic kinase Cdk1p/Cdc2p/Cdc28p and the casein kinase Hrr25p. Hrr25p was identified by screening 119 yeast kinases for their ability to phosphorylate Nup53p in vitro. Conditional alleles of Hrr25p support the conclusion that Hrr25p phosphorylates Nup53p in vivo. We further demonstrated using solution binding and affinity purification assays, that Hrr25p directly binds Nup53p in an interaction that is destabilized by the phosphorylation of Nup53p. Consistent with this observation, we observed that Hrr25p moves between distinct locations in the cell during the cell cycle including the nucleus, the cortex of the emerging bud and the spindle pole bodies. Cdk1p also contributes to Nup53p phosphorylation as specific inhibition of Cdk1p or mutation of Cdk1p consensus sites partially blocked its phosphorylation. The ability of nup53 alleles containing Cdk1p site mutations to complement synthetic defects of nup53 Delta nup170 Delta strains is linked to a function for Nup53p in the spindle assembly checkpoint.

New ways to skin a kap: mechanisms for controlling nuclear transport.

Transport between the nucleus and the cytoplasm occurs through large macromolecular assemblies called nuclear pore complexes (NPCs). The NPC is traditionally viewed as a passive structure whose primary role is to provide an interface for the soluble transport machinery, the karyopherins and their cargos, to move molecules between these compartments. Recent work has challenged this view of the NPC and provides support for a dynamic structure that can modify its architecture to actively regulate nuclear transport.

Cell cycle regulated transport controlled by alterations in the nuclear pore complex.

Eukaryotic cells have developed mechanisms for regulating the nuclear transport of macromolecules that control various cellular events including movement through defined stages of the cell cycle. In yeast cells, where the nuclear envelope remains intact throughout the cell cycle, these transport regulatory mechanisms must also function during mitosis. We have uncovered a mechanism for regulating transport that is controlled by M phase specific molecular rearrangements in the nuclear pore complex (NPC). These changes allow a transport inhibitory nucleoporin, Nup53p, to bind the karyopherin Kap121p specifically during mitosis, slowing its movement through the NPC and inducing cargo release. Yeast strains that possess defects in the function of Kap121p or the fidelity of the inhibitory pathway are delayed in mitosis. We propose that fluctuations in Kap121p transport mediated by the NPC contribute to controlling the subcellular distribution of molecules that direct progression through mitosis.