Retinoblastoma protein and anaphase-promoting complex physically interact and functionally cooperate during cell-cycle exit

The retinoblastoma protein (pRB) negatively regulates the progression from G1 to S phase of the cell cycle, in part, by repressing E2F-dependent transcription. pRB also possesses E2F-independent functions that contribute to cell-cycle control--for example, during pRB-mediated cell-cycle arrest pRB associates with Skp2, the F-box protein of the Skp1-Cullin-F-box protein (SCF) E3 ubiquitin ligase complex, and promotes the stability of the cyclin-dependent kinase-inhibitor p27(Kip1) through an unknown mechanism. Degradation of p27(Kip1) is mediated by ubiquitin-dependent targeting of p27(Kip1) by SCF -Skp2 (ref. 4). Here, we report a novel interaction between pRB and the anaphase-promoting complex/cyclosome (APC/C) that controls p27(Kip1) stability by targeting Skp2 for ubiquitin-mediated degradation. Cdh1, an activator of APC/C, not only interacts with pRB but is also required for a pRB-induced cell-cycle arrest. The results reveal an unexpected physical convergence between the pRB tumour-suppressor protein and E3 ligase complexes, and raise the possibility that pRB may direct APC/C to additional targets during pRB-mediated cell-cycle exit.

Precise destruction: an emerging picture of the APC

Cell cycle transitions are often accompanied by the degradation of regulatory molecules. Targeting proteins to the proteasome for degradation is accomplished by the covalent addition of ubiquitin chains. The specificity of this pathway is largely dictated by a set of enzymes called ubiquitin ligases (or E3s). The anaphase-promoting complex (or APC) is a ubiquitin ligase that has a particularly prominent role in regulating cell cycle progression. To date, the APC is the most complicated member of the RING/cullin family of multisubunit E3s. It includes at least 13 core subunits and three related adaptors. A combination of biochemical, genetic, and structural approaches are now shedding light on the enzymology of the APC. This review will focus on these data, drawing parallels with related ubiquitin ligases.

Characterization of multiple multivesicular body sorting determinants within Sna3: a role for the ubiquitin ligase Rsp5

A subset of proteins that transit the endosomal system are directed into the intralumenal vesicles of multivesicular bodies (MVBs). MVB formation is critical for a variety of cellular functions including receptor down-regulation, viral budding, antigen presentation, and the generation of lysosome-related organelles. Entry of transmembrane proteins into the intralumenal vesicles of a MVB is a highly regulated process that is positively modulated by covalent modification of cargoes with ubiquitin. To identify additional MVB sorting signals, we examined the previously described ubiquitination-independent MVB cargo Sna3. Although Sna3 ubiquitination is not essential, Sna3 MVB sorting is positively modulated by its ubiquitination. Examination of MVB sorting determinants within a form of Sna3 lacking all lysine residues identified two critical regions: an amino-terminal tyrosine-containing region and a carboxyl-terminal PPAY motif. This PPAY motif interacts with the WW domains of the ubiquitin ligase Rsp5, and mutations in either the WW or, surprisingly, the HECT domains of Rsp5 negatively impacted MVB targeting of lysine-minus Sna3. These data indicate that Rsp5 function is required for MVB targeting of Sna3 in a capacity beyond cargo ubiquitination. These results uncover a series of determinants impacting Sna3 MVB sorting, including unexpected roles for Rsp5.

Siah-1 facilitates ubiquitination and degradation of factor inhibiting HIF-1alpha (FIH)

Hypoxia-inducible factor-1alpha (HIF-1alpha) has a central role in neuroprotective responses to hypoxia in the brain. Hydroxylation of HIF-1alpha by prolyl-hydroxylase PHD and aspargynyl-hydroxylase FIH (factor inhibiting HIF-1alpha) causes proteasomal degradation and transcriptional inhibition of HIF-1alpha. Siah ubiquitin ligases regulate the abundance of PHD via targeting for proteasomal degradation. The present study identified Siah-1 as a binding partner for another hydroxylase FIH. Siah-1 and FIH coimmunoprecipitated each other in mammalian cells. Siah-1 was found both to interact with the JmjC domain of FIH through its substrate-binding domain and to specifically ubiquitinate FIH via its RING finger domain. Siah-1 facilitated FIH degradation via the ubiquitin-proteasome pathway under hypoxic conditions. Such findings suggest that Siah ubiquitin ligases might play a role as up-stream regulators of both hydroxylases for HIF-1alpha, i.e., PHD and FIH, by targeting them for proteasomal degradation, leading to increased HIF-1alpha abundance, and transcriptional activity in hypoxia.

Peroxisomal matrix protein receptor ubiquitination and recycling

The peroxisomal targeting signal type1 (PTS1) receptor Pex5 is required for the peroxisomal targeting of most matrix proteins. Pex5 recognises target proteins in the cytosol and directs them to the peroxisomal membrane where cargo is released into the matrix, and the receptor shuttles back to the cytosol. Recently, it has become evident that the membrane-bound Pex5 can be modified by mono- and polyubiquitination. This review summarises recent results on Pex5 ubiquitination and on the role of the AAA peroxins Pex1 and Pex6 as dislocases required for the release of Pex5 from the membrane to the cytosol where the receptor is either degraded by proteasomes or made available for another round of protein import into peroxisomes.

Human PIF helicase is cell cycle regulated and associates with telomerase

The evolutionarily conserved PIF1 DNA helicase family is important for the maintenance of genome stability in the yeast, Saccharomyces cerevisiae. There are two PIF1 family helicases in S. cerevisiae, Pif1p and Rrm3p that both possess 5'-->3' DNA helicase activity but maintain unique functions in telomerase regulation and semi-conservative DNA replication. Database analysis shows that the PIF1 helicase family is represented by a single homologue in higher eukaryotes. To analyze the function of PIF1 homologues in mammals, we cloned the full length human PIF (hPIF) cDNA. Comparison of hPIF with its S. cerevisiae homologues showed that human PIF is equally similar to Pif1p and Rrm3p. Human PIF was expressed at low levels in a variety of tissues and immunofluorescence analysis showed that ectopic hPIF was localized to nuclear foci. hPIF was expressed in late S/G2 phase of the cell cycle and this cell cycle regulated abundance was conferred by both cell cycle regulated mRNA accumulation and ubiquitin-mediated degradation. Furthermore, hPIF is likely a target of the anaphase promoting complex/cyclosome as its abundance was decreased when an activator of the APC/C was overexpressed. Finally, antibodies against hPIF immunoprecipitated telomerase activity from human cell lines, and we have observed a physical interaction between hPIF and the catalytic subunit of telomerase, hTERT. Our data suggest that human PIF, like S. cerevisiae Pif1p, plays a role in telomerase regulation.

SCFCdc4-mediated degradation of the Hac1p transcription factor regulates the unfolded protein response in Saccharomyces cerevisiae

The Saccharomyces cerevisiae basic leucine zipper transcription factor Hac1p is synthesized in response to the accumulation of unfolded polypeptides in the lumen of the endoplasmic reticulum (ER), and it is responsible for up-regulation of approximately 5% of all yeast genes, including ER-resident chaperones and protein-folding catalysts. Hac1p is one of the most short-lived yeast proteins, having a half-life of approximately 1.5 min. Here, we have shown that Hac1p harbors a functional PEST degron and that degradation of Hac1p by the proteasome involves the E2 ubiquitin-conjugating enzyme Ubc3/Cdc34p and the SCF(Cdc4) E3 complex. Consistent with the known nuclear localization of Cdc4p, rapid degradation of Hac1p requires the presence of a functional nuclear localization sequence, which we demonstrated to involve basic residues in the sequence (29)RKRAKTK(35). Two-hybrid analysis demonstrated that the PEST-dependent interaction of Hac1p with Cdc4p requires Ser146 and Ser149. Turnover of Hac1p may be dependent on transcription because it is inhibited in cell mutants lacking Srb10 kinase, a component of the SRB/mediator module of the RNA polymerase II holoenzyme. Stabilization of Hac1p by point mutation or deletion, or as the consequence of defects in components of the degradation pathway, results in increased unfolded protein response element-dependent transcription and improved cell viability under ER stress conditions.

Molecular dissection of the APC/C inhibitor Rca1 shows a novel F-box-dependent function

Rca1 (regulator of Cyclin A)/Emi (early mitotic inhibitor) proteins are essential inhibitors of the anaphase-promoting complex/cyclosome (APC/C). In Drosophila, Rca1 is required during G2 to prevent premature cyclin degradation by the Fizzy-related (Fzr)-dependent APC/C activity. Here, we present a structure and function analysis of Rca1 showing that a carboxy-terminal fragment is sufficient for APC/C inhibition. Rca1/Emi proteins contain a conserved F-box and interact with components of the Skp-Cullin-F-box (SCF) complex. So far, no function has been ascribed to this domain. We find that the F-box of Rca1 is dispensable for APC/C-Fzr inhibition during G2. Nevertheless, we show that Rca1 has an additional function at the G1-S transition, which requires the F-box. Overexpression of Rca1 accelerates the G1-S transition in an F-box-dependent manner. Conversely, S-phase entry is delayed in cells in which endogenous Rca1 is replaced by a transgene lacking the F-box. We propose that Rca1 acts as an F-box protein in an as yet uncharacterized SCF complex, which promotes S-phase entry.

Ubiquitin-binding domains

The covalent modification of proteins by ubiquitination is a major regulatory mechanism of protein degradation and quality control, endocytosis, vesicular trafficking, cell-cycle control, stress response, DNA repair, growth-factor signalling, transcription, gene silencing and other areas of biology. A class of specific ubiquitin-binding domains mediates most of the effects of protein ubiquitination. The known membership of this group has expanded rapidly and now includes at least sixteen domains: UBA, UIM, MIU, DUIM, CUE, GAT, NZF, A20 ZnF, UBP ZnF, UBZ, Ubc, UEV, UBM, GLUE, Jab1/MPN and PFU. The structures of many of the complexes with mono-ubiquitin have been determined, revealing interactions with multiple surfaces on ubiquitin. Inroads into understanding polyubiquitin specificity have been made for two UBA domains, whose structures have been characterized in complex with Lys48-linked di-ubiquitin. Several ubiquitin-binding domains, including the UIM, CUE and A20 ZnF (zinc finger) domains, promote auto-ubiquitination, which regulates the activity of proteins that contain them. At least one of these domains, the A20 ZnF, acts as a ubiquitin ligase by recruiting a ubiquitin-ubiquitin-conjugating enzyme thiolester adduct in a process that depends on the ubiquitin-binding activity of the A20 ZnF. The affinities of the mono-ubiquitin-binding interactions of these domains span a wide range, but are most commonly weak, with Kd>100 microM. The weak interactions between individual domains and mono-ubiquitin are leveraged into physiologically relevant high-affinity interactions via several mechanisms: ubiquitin polymerization, modification multiplicity, oligomerization of ubiquitinated proteins and binding domain proteins, tandem-binding domains, binding domains with multiple ubiquitin-binding sites and co-operativity between ubiquitin binding and binding through other domains to phospholipids and small G-proteins.

The anaphase-promoting complex is required in both dividing and quiescent cells during zebrafish development

The anaphase-promoting complex/cyclosome (APC/C) regulates multiple stages of the cell cycle, most prominently mitosis. We describe zebrafish with mutations in two APC/C subunits, Cdc16 and Cdc26, whose phenotypes reveal a multifaceted set of defects resulting from the gradual depletion of the APC/C. First, loss of the APC/C in dividing cells results in mitotic arrest, followed by apoptosis. This defect becomes detectable in different organs at different larval ages, because the subunits of the APC/C are maternally deposited, are unusually stable, and are depleted at uneven rates in different tissues. Second, loss of the APC/C in quiescent or differentiated cells results in improper re-entry into the cell cycle, again in an apparently tissue-specific manner. This study is the first demonstration of both functions of the APC/C in a vertebrate organism and also provides an illustration of the surprisingly complex effects that essential, maternally supplied factors can have on the growing animal over a period of 10 days or longer.

Endothelial cell loss is not a major cause of neuronal and glial cell death following contusion injury of the spinal cord

Contusion of the spinal cord causes an immediate local loss of neurons and disruption of vasculature; additional loss continues thereafter. To explore the possibility of a causal link between delayed endothelial cell (EC) death and secondary neural cell loss, we evaluated neural and endothelial cell survival, and measured inflammatory cell infiltration, at times up to 48 h after contusion injury to the adult rat thoracic spinal cord. Female Fischer rats (200 g), subjected to moderate (10 g x 12.5 mm) weight drop injuries by the MASCIS (NYU) impactor, were analyzed at 15 min and at 1, 8, 24 and 48 h. ECs, neurons, astrocytes, oligodendrocytes, neutrophils and activated macrophages/microglia were counted in transverse sections. At the injury site, 90% of all neurons died within 48 h of injury; no medium-large diameter neurons survived beyond 48 h. EC death occurred with kinetics similar to glial cell death. Because, in the injury site, most cell death occurred before 8 h, it preceded inflammatory cell infiltration. Three millimeters rostral and caudal to the injury epicenter neuronal numbers were stable for 8 h, and a sharp decrease in neuronal numbers beginning at 8 h strongly correlated with the onset of inflammatory cell infiltration. Glial and blood vessel numbers remained relatively stable in these areas up to 48 h. These results suggest that the loss of ECs during the first 48 h after a contusion injury is not a major cause of neuronal and glial cell death and, in tissue adjacent to the epicenter, inflammatory cell infiltration leads to neuronal loss.

Pds1/Esp1-dependent and -independent sister chromatid separation in mutants defective for protein phosphatase 2A

Spindle disruption or DNA damage prevents sister chromatid separation through the activation of checkpoint pathways that inhibit anaphase entry by stabilizing the anaphase inhibitor Pds1. Mutation of CDC55, which encodes a B regulatory subunit of protein phosphatase 2A (PP2A), results in precocious sister chromatid separation when spindle is disrupted. Here we report that decreased Pds1 levels in Deltacdc55 mutants contribute to sister chromatid separation in the presence of nocodazole, a microtubule-depolymerizing drug. However, in the presence of DNA damage, Deltacdc55 mutant cells separate sister chromatids without noticeable decrease of Pds1 or cohesin Mcd1/Scc1 levels. Further analysis demonstrates that Deltacdc55 mutants lose cohesion along the entire chromosomes when the spindle is disrupted. In contrast, separation of sister chromatids is limited to the centromeric regions in Deltacdc55 cells after DNA damage. Moreover, mutation of TPD3, which encodes the A regulatory subunit of PP2A, also results in sister chromatid separation in DNA- or spindle-damage-arrested cells. These data suggest that PP2A regulates sister chromatid cohesion in Pds1-dependent and -independent manners.

Modeling dual pathways for the metazoan spindle assembly checkpoint

Using computational modeling, we investigate mechanisms of signal transduction. We focus on the spindle assembly checkpoint, where a single unattached kinetochore is able to signal to prevent cell cycle progression. The inhibitory signal switches off rapidly once spindle microtubules have attached to all kinetochores. This requirement tightly constrains the possible mechanisms. Here we investigate two possible mechanisms for spindle checkpoint operation in metazoan cells, both supported by recent experiments. The first involves the free diffusion and sequestration of cell cycle regulators. This mechanism is severely constrained both by experimental fluorescence recovery data and by the large volumes involved in open mitosis in metazoan cells. By using a simple mathematical analysis and computer simulation, we find that this mechanism can generate the inhibition found in experiment but likely requires a two-stage signal amplification cascade. The second mechanism involves spatial gradients of a short-lived inhibitory signal that propagates first by diffusion but then primarily by active transport along spindle microtubules. We propose that both mechanisms may be operative in the metazoan spindle assembly checkpoint, with either able to trigger anaphase onset even without support from the other pathway.

Acm1 is a negative regulator of the CDH1-dependent anaphase-promoting complex/cyclosome in budding yeast

Cdh1 is a coactivator of the anaphase-promoting complex/cyclosome (APC/C) and contributes to mitotic exit and G1 maintenance by facilitating the polyubiquitination and subsequent proteolysis of specific substrates. Here, we report that budding yeast Cdh1 is a component of a cell cycle-regulated complex that includes the 14-3-3 homologs Bmh1 and Bmh2 and a previously uncharacterized protein, which we name Acm1 (APC/CCdh1 modulator 1). Association of Cdh1 with Bmh1 and Bmh2 requires Acm1, and the Acm1 protein is cell cycle regulated, appearing late in G1 and disappearing in late M. In acm1Delta strains, Cdh1 localization to the bud neck and association with two substrates, Clb2 and Hsl1, were strongly enhanced. Several lines of evidence suggest that Acm1 can suppress APC/CCdh1-mediated proteolysis of mitotic cyclins. First, overexpression of Acm1 fully restored viability to cells expressing toxic levels of Cdh1 or a constitutively active Cdh1 mutant lacking inhibitory phosphorylation sites. Second, overexpression of Acm1 was toxic in sic1Delta cells. Third, ACM1 deletion exacerbated a low-penetrance elongated-bud phenotype caused by modest overexpression of Cdh1. This bud elongation was independent of the morphogenesis checkpoint, and the combination of acm1Delta and hsl1Delta resulted in a dramatic enhancement of bud elongation and G2/M delay. Effects on bud elongation were attenuated when Cdh1 was replaced with a mutant lacking the C-terminal IR dipeptide, suggesting that APC/C-dependent proteolysis is required for this phenotype. We propose that Acm1 and Bmh1/Bmh2 constitute a specialized inhibitor of APC/CCdh1.

The Cdc34/SCF ubiquitination complex mediates Saccharomyces cerevisiae cell wall integrity

To identify novel functions for the Cdc34/SCF ubiquitination complex, we analyzed genomewide transcriptional profiles of cdc53-1 and cdc34-2 Saccharomyces cerevisiae mutants. This analysis revealed altered expression for several gene families, including genes involved in the regulation of cell wall organization and biosynthesis. This led us to uncover a role for the Cdc34/SCF complex in the regulation of cell wall integrity. In support of this, cdc53-1 and cdc34-2 mutants exhibit phenotypes characteristic of cell wall integrity mutants, such as SDS sensitivity and temperature-sensitive suppression by osmotic stabilizers. Examination of these mutants revealed defects in their induction of Slt2 phosphorylation, indicating defects in Pkc1-Slt2 MAPK signaling. Consistent with this, synthetic genetic interactions were observed between the genes encoding the Cdc34/SCF complex and key components of the Pck1-Slt2 MAPK pathway. Further analysis revealed that Cdc34/SCF mutants have reduced levels of active Rho1, suggesting that these defects stem from the deregulated activity of the Rho1 GTPase. Altering the activity of Rho1 via manipulation of the Rho1-GAPs LRG1 or SAC7 affected Cdc34/SCF mutant growth. Strikingly, however, deletion of LRG1 rescued the growth defects associated with Cdc34/SCF mutants, whereas deletion of SAC7 enhanced these defects. Given the differential roles that these GAPs play in the regulation of Rho1, these observations indicate the importance of coordinating Cdc34/SCF activity with specific Rho1 functions.

The deubiquitinating enzyme Ubp2 modulates Rsp5-dependent Lys63-linked polyubiquitin conjugates in Saccharomyces cerevisiae

The functions of Lys(63)-linked polyubiquitin chains are poorly understood, as are the enzymes that specifically generate Lys(63)-linked conjugates. Rsp5 is a HECT (homologous to E6AP C terminus) ubiquitin ligase involved in numerous processes, and an associated deubiquitinating enzyme, Ubp2, modulates its activity. A dramatic increase in Lys(63)-linked conjugates was observed in ubp2Delta cells. The formation of these was Rsp5-dependent, and ubp2Delta phenotypes could be suppressed by prevention of formation of Lys(63) conjugates. Cell wall integrity was impaired in rsp5-1 cells and in cells defective in Lys(63)-polyubiquitination, as assayed by calcofluor white sensitivity, and ubp2Delta and rup1Delta mutants suppressed the calcofluor white sensitivity of rsp5-1. A large fraction of the Lys(63) conjugates in ubp2Delta cells bound to Rsp5, and a proteomics approach was used to identify Rsp5 substrates subject to Ubp2 regulation. Two closely related proteins, Csr2 and Ecm21, were among the identified proteins. Both were efficiently Lys(63)-polyubiquitinated by Rsp5 and deubiquitinated by Ubp2. Together, these results indicate that Ubp2 modulates Lys(63)-polyubiquitination of Rsp5 substrates in vivo, including ubiquitination of two newly identified Rsp5 substrates.

AML1/RUNX1 phosphorylation by cyclin-dependent kinases regulates the degradation of AML1/RUNX1 by the anaphase-promoting complex

AML1 (RUNX1) regulates hematopoiesis, angiogenesis, muscle function, and neurogenesis. Previous studies have shown that phosphorylation of AML1, particularly at serines 276 and 303, affects its transcriptional activation. Here, we report that phosphorylation of AML1 serines 276 and 303 can be blocked in vivo by inhibitors of the cyclin-dependent kinases (CDKs) Cdk1 and Cdk2. Furthermore, these residues can be phosphorylated in vitro by purified Cdk1/cyclin B and Cdk2/cyclin A. Mutant AML1 protein which cannot be phosphorylated at these sites (AML1-4A) is more stable than wild-type AML1. AML-4A is resistant to degradation mediated by Cdc20, one of the substrate-targeting subunits of the anaphase-promoting complex (APC). However, Cdh1, another targeting subunit used by the APC, can mediate the degradation of AML1-4A. A phospho-mimic protein, AML1-4D, can be targeted by Cdc20 or Cdh1. These observations suggest that both Cdc20 and Cdh1 can target AML1 for degradation by the APC but that AML1 phosphorylation may affect degradation mediated by Cdc20-APC to a greater degree.

Ubiquitin and ubiquitin-like proteins in cancer pathogenesis

Ubiquitin and ubiquitin-like proteins (Ubls) are signalling messengers that control many cellular functions, such as cell proliferation, apoptosis, the cell cycle and DNA repair. It is becoming apparent that the deregulation of ubiquitin pathways results in the development of human diseases, including many types of tumours. Here we summarize the common principles and specific features of ubiquitin and Ubls in the regulation of cancer-relevant pathways, and discuss new strategies to target ubiquitin signalling in drug discovery.

The Rho-mDia1 pathway regulates cell polarity and focal adhesion turnover in migrating cells through mobilizing Apc and c-Src

Directed cell migration requires cell polarization and adhesion turnover, in which the actin cytoskeleton and microtubules work critically. The Rho GTPases induce specific types of actin cytoskeleton and regulate microtubule dynamics. In migrating cells, Cdc42 regulates cell polarity and Rac works in membrane protrusion. However, the role of Rho in migration is little known. Rho acts on two major effectors, ROCK and mDia1, among which mDia1 produces straight actin filaments and aligns microtubules. Here we depleted mDia1 by RNA interference and found that mDia1 depletion impaired directed migration of rat C6 glioma cells by inhibiting both cell polarization and adhesion turnover. Apc and active Cdc42, which work together for cell polarization, localized in the front of migrating cells, while active c-Src, which regulates adhesion turnover, localized in focal adhesions. mDia1 depletion impaired localization of these molecules at their respective sites. Conversely, expression of active mDia1 facilitated microtubule-dependent accumulation of Apc and active Cdc42 in the polar ends of the cells and actin-dependent recruitment of c-Src in adhesions. Thus, the Rho-mDia1 pathway regulates polarization and adhesion turnover by aligning microtubules and actin filaments and delivering Apc/Cdc42 and c-Src to their respective sites of action.

The formin mDia regulates GSK3beta through novel PKCs to promote microtubule stabilization but not MTOC reorientation in migrating fibroblasts

In migrating cells, external signals polarize the microtubule (MT) cytoskeleton by stimulating the formation of oriented, stabilized MTs and inducing the reorientation of the MT organizing center (MTOC). Glycogen synthase kinase 3beta (GSK3beta) has been implicated in each of these processes, although whether it regulates both processes in a single system and how its activity is regulated are unclear. We examined these issues in wound-edge, serum-starved NIH 3T3 fibroblasts where MT stabilization and MTOC reorientation are triggered by lysophosphatidic acid (LPA), but are regulated independently by distinct Rho GTPase-signaling pathways. In the absence of other treatments, the GSK3beta inhibitors, LiCl or SB216763, induced the formation of stable MTs, but not MTOC reorientation, in starved fibroblasts. Overexpression of GSK3beta in starved fibroblasts inhibited LPA-induced stable MTs without inhibiting MTOC reorientation. Analysis of factors involved in stable MT formation (Rho, mDia, and EB1) showed that GSK3beta functioned upstream of EB1, but downstream of Rho-mDia. mDia was both necessary and sufficient for inducing stable MTs and for up-regulating GSK3beta phosphorylation on Ser9, an inhibitory site. mDia appears to regulate GSK3beta through novel class PKCs because PKC inhibitors and dominant negative constructs of novel PKC isoforms prevented phosphorylation of GSK3beta Ser9 and stable MT formation. Novel PKCs also interacted with mDia in vivo and in vitro. These results identify a new activity for the formin mDia in regulating GSK3beta through novel PKCs and implicate novel PKCs as new factors in the MT stabilization pathway.

Substrate-mediated remodeling of methionine transport by multiple ubiquitin-dependent mechanisms in yeast cells

Plasma membrane transport of single amino-acid methionine in yeast is shown to be mediated by at least seven different permeases whose activities are transcriptionaly and post-transcriptionaly regulated by different ubiquitin-dependent mechanisms. Upon high extracellular methionine exposure, three methionine-permease genes are repressed while four others are induced. SCF(Met30), SCF(Grr1) and Rsp5 ubiquitin ligases are the key actors of the ubiquitin-dependent remodeling of methionine transport. In addition to regulating the activity of Met4, the SCF(Met30) ubiquitin ligase is shown to convey an intracellular signal to a membrane initiated signaling pathway by controlling the nuclear concentration of the Stp1 transcription factor. By coupling intra- and extracellular metabolite sensing, SCF(Met30) thus allows yeast cells to accurately adjust the intermediary sulfur metabolism to the growth conditions. The multiple ubiquitin-dependent mechanisms that function in methionine transport regulation further exemplify the pervasive role of ubiquitin in the adaptation of single-cell organisms to environmental modifications.

Ubiquitin pathway in VHL cancer syndrome

The physiologic response to changes in cellular oxygen tension is ultimately governed by a heterodimeric transcription factor called hypoxia-inducible factor (HIF), which, in adaptation to compromised oxygen availability, transactivates a myriad of genes, including those responsible for de novo vascularization, production of oxygen-carrying red blood cells, and anaerobic metabolism. Accumulation of HIF is observed in most types of solid tumors and is frequently associated with poor prognosis and disease progression, underscoring the importance and relevance of HIF in cancer. The protein stability and, thereby, the activity of HIF are principally regulated by the von Hippel-Lindau (VHL) tumor suppressor-containing E3 ubiquitin ligase complex (ECV) that targets the catalytic subunit HIFalpha for oxygen-dependent ubiquitin-mediated destruction. Individuals who inherit germline VHL mutation develop VHL disease, which is characterized by the development of hypervascular tumors in multiple yet specific organs. This review will examine recent progress in our understanding of the molecular mechanisms governing the function of ECV and the significance of consequential regulation of HIF in oncogenesis.

Balance of Yin and Yang: ubiquitylation-mediated regulation of p53 and c-Myc

Protein ubiquitylation has been demonstrated to play a vital role not only in mediating protein turnover but also in modulating protein activity. The stability and activity of the tumor suppressor p53 and of the oncoprotein c-Myc are no exception. Both are regulated through independent ubiquitylation by several E3 ubiquitin ligases. Interestingly, p53 and c-Myc are functionally connected by some of these E3 enzymes and their regulator ARF, although these proteins play opposite roles in controlling cell growth and proliferation. The balance of this complex ubiquitylation network and its disruption during oncogenesis will be the topics of this review.

Neuronal polarity is regulated by glycogen synthase kinase-3 (GSK-3beta) independently of Akt/PKB serine phosphorylation

An essential step during the development of hippocampal neurons is the polarised outgrowth of a single axon. Recently, it has been suggested that inhibition of glycogen synthase kinase-3beta (GSK-3beta) via Akt/PKB-dependent phosphorylation of Ser9, specifically at the tip of the presumptive axon, is required for selective axonal outgrowth. We now report that, by using neurons from double knock-in mice in which Ser9 and Ser21 of the two GSK-3beta isoforms have been replaced by Ala, polarity develops independently of phosphorylation at these sites. Nevertheless, global inhibition of GSK-3beta disturbs polarity development by leading to the formation of multiple axon-like processes in both control and knock-in neurons. This unpolarised outgrowth is accompanied by the symmetric delivery of membrane components to all neurites. Finally, the adenomatous polyposis coli (APC) protein accumulates at the tip of one neurite before and during axon elongation, but global inhibition of GSK-3beta leads to APC protein accumulation in all neurites. We conclude that GSK-3beta inhibition promotes the development of neuronal polarity, but that this is not mediated by Akt/PKB-dependent phosphorylation.