PEST sequences mediate heat shock factor 2 turnover by interacting with the Cul3 subunit of the Cul3-RING ubiquitin ligase

Cullin-RING ubiquitin ligases promote the polyubiquitination and degradation of many important cellular proteins, which previous studies indicated can be targeted for degradation via interaction with BTB domain-containing subunits of this E3 ligase complex. PEST domains are known to promote the degradation of proteins that contain them. However, the molecular mechanism by which PEST sequences promote degradation of these proteins is not understood. Here we show that the PEST sequences of a short-lived protein called HSF2 interact with Cullin3, a subunit of a Cullin-RING E3 ubiquitin ligase, and that this interaction mediates the Cul3-dependent ubiquitination and degradation of HSF2. These results indicate how, at the molecular level, PEST sequences can promote the proteolysis of proteins that contain them. They also expand understanding of the mechanisms by which substrates can be recruited to Cullin-RING E3 ubiquitin ligases to include interactions between PEST sequences and Cul3.     

Deletion of the Cul1 gene in mice causes arrest in early embryogenesis and accumulation of cyclin E.

The stability of many proteins is controlled by the ubiquitin proteolytic system, which recognizes specific substrates through the action of E3 ubiquitin ligases [1]. The SCFs are a recently described class of ubiquitin ligase that target a number of cell cycle regulators and other proteins for degradation in both yeast and mammalian cells [2] [3] [4] [5] [6]. Each SCF complex is composed of the core protein subunits Skp1, Rbx1 and Cul1 (known as Cdc53 in yeast), and substrate-specific adaptor subunits called F-box proteins [2] [3] [4]. To understand the physiological role of SCF complexes in mammalian cells, we generated mice carrying a deletion in the Cul1 gene. Cul1(-/-) embryos arrested around embryonic day 6.5 (E6.5) before the onset of gastrulation. In all cells of the mutant embryos, cyclin E protein, but not mRNA, was highly elevated. Outgrowths of Cul1(-/-) blastocysts had limited proliferative capacity in vitro and accumulated cyclin E in all cells. Within Cul1(-/-) blastocyst cultures, trophoblast giant cells continued to endocycle despite the elevated cyclin E levels. These results suggest that cyclin E abundance is controlled by SCF activity, possibly through SCF-dependent degradation of cyclin E.     

Regulation of the cyclin D3 promoter by E2F1

We have previously demonstrated that ectopic expression of E2F1 is sufficient to drive quiescent cells into S phase and that E2F1 expression can contribute to oncogenic transformation. Key target genes in this process include master regulators of the cell cycle, such as cyclin E, which regulates G(1) progression, and cyclin A, which is required for the initiation of DNA synthesis. In the present work, we present novel evidence that a second G(1) cyclin, cyclin D3, is also potently activated by E2F1. First, an estrogen receptor-E2F1 fusion protein (ER-E2F1) potently activates the endogenous cyclin D3 mRNA upon treatment with 4-hydroxytamoxifen, which induces nuclear accumulation of the otherwise cytosolic fusion protein. Furthermore, trans-activation of cyclin D3 by ER-E2F1 occurs even in the presence of the protein synthesis inhibitor cycloheximide and thus appears direct. Second, all of the growth-stimulatory members of the E2F family (E2F1, -2, and -3A) potently activate a cyclin D3 promoter reporter, whereas growth-restraining members of the family (E2F4, -5, and -6) have little effect. Third, recombinant E2F1 binds with high affinity to the cyclin D3 promoter in vitro. Fourth, chromatin immunoprecipitation assays demonstrate that endogenous E2F1 is associated with the cyclin D3 promoter in vivo. Finally, mapping experiments localize the essential E2F regulatory element of the cyclin D3 promoter to a noncanonical E2F site in the promoter between nucleotides -143 and -135 relative to the initiating methionine codon. We conclude that in addition to cyclins E and A, E2F family members can also activate one member of the D-type cyclins, further contributing to the ability of the stimulatory E2F family members to drive cellular proliferation.     

Rescue of cyclin D1 deficiency by knockin cyclin E.

D-type cyclins and cyclin E represent two very distinct classes of mammalian G1 cyclins. We have generated a mouse strain in which the coding sequences of the cyclin D1 gene (Ccnd1) have been deleted and replaced by those of human cyclin E (CCNE). In the tissues and cells of these mice, the expression pattern of human cyclin E faithfully reproduces that normally associated with mouse cyclin D1. The replacement of cyclin D1 with cyclin E rescues all phenotypic manifestations of cyclin D1 deficiency and restores normal development in cyclin D1-dependent tissues. Thus, cyclin E can functionally replace cyclin D1. Our analyses suggest that cyclin E is the major downstream target of cyclin D1.     

Multisite phosphorylation by Cdk2 and GSK3 controls cyclin E degradation

Autophosphorylation-triggered ubiquitination has been proposed to be the major pathway regulating cyclin E protein abundance: phosphorylation of cyclin E on T380 by its associated CDK allows binding to the receptor subunit, Fbw7, of the SCFFbw7 ubiquitin ligase. We have tested this model in vivo and found it to be an inadequate representation of the pathways that regulate cyclin E degradation. We show that assembly of cyclin E into cyclin E-Cdk2 complexes is required in vivo for turnover by the Fbw7 pathway; that Cdk2 activity is required for cyclin E turnover in vivo because it phosphorylates S384; that phosphorylation of T380 in vivo does not require Cdk2 and is mediated primarily by GSK3; and that two additional phosphorylation sites, T62 and S372, are also required for turnover. Thus, cyclin E turnover is controlled by multiple biological inputs and cannot be understood in terms of autophosphorylation alone.     

Structural Basis for Cul3 Protein Assembly with the BTB-Kelch Family of E3 Ubiquitin Ligases

Cullin-RING ligases are multisubunit E3 ubiquitin ligases that recruit substrate-specific adaptors to catalyze protein ubiquitylation. Cul3-based Cullin-RING ligases are uniquely associated with BTB adaptors that incorporate homodimerization, Cul3 assembly, and substrate recognition into a single multidomain protein, of which the best known are BTB-BACK-Kelch domain proteins, including KEAP1. Cul3 assembly requires a BTB protein “3-box” motif, analogous to the F-box and SOCS box motifs of other Cullin-based E3s. To define the molecular basis for this assembly and the overall architecture of the E3, we determined the crystal structures of the BTB-BACK domains of KLHL11 both alone and in complex with Cul3, along with the Kelch domain structures of KLHL2 (Mayven), KLHL7, KLHL12, and KBTBD5. We show that Cul3 interaction is dependent on a unique N-terminal extension sequence that packs against the 3-box in a hydrophobic groove centrally located between the BTB and BACK domains. Deletion of this N-terminal region results in a 30-fold loss in affinity. The presented data offer a model for the quaternary assembly of this E3 class that supports the bivalent capture of Nrf2 and reveals potential new sites for E3 inhibitor design.     

Morning pulse of prolactin maintains seasonal quiescence in the tammar, Macropus eugenii

The pulse of prolactin, present in female tammars kept in an inhibitory photoperiod and absent 5 days after a change to a stimulatory photoperiod, was artificially maintained during stimulatory photoperiods (Exps 1A and 1B) or abolished during an inhibitory photoperiod (Exp. 2). In Exp. 1A, thyrotrophin-releasing hormone (TRH) was injected i.v. for 8 consecutive mornings and in Exp. 1B ovine prolactin was injected i.v. for 10 consecutive mornings beginning on the day the photoperiod was changed from 15L:9D to 12L:12D. In Exp. 1A, in 5 of 6 control females the prolactin pulse was present until the 3rd morning after the lighting was changed, and absent thereafter. In both experiments the early progesterone peak and the interval to birth in the treated females was significantly delayed compared to the control females; reactivation of the CL did not occur until treatment ceased. In Exp. 2, for 5 consecutive mornings, before the prolactin pulse was expected to occur, bromocriptine was injected i.m. In 2/4 treated animals the prolactin pulse was abolished and these animals showed an early progesterone peak and gave birth 28 and 29 days after treatment began. The remaining 2 animals and the controls (N = 4) did not show reactivation until after they experienced a stimulatory photoperiod. The results indicate that the prolactin pulse is the critical factor in the maintenance of the inhibition of the CL of the tammar during seasonal quiescence.     

BTB Protein KLHL12 targets the dopamine D4 receptor for ubiquitination by a Cul3-based E3 ligase

Dopamine receptors belong to the superfamily of G-protein-coupled receptors and are subdivided into D1-type (D1 and D5) and D2-type (D2, D3, and D4) receptors. The D4 receptor has a remarkable polymorphism in its third intracellular loop, which is under intensive investigation and which has been associated with, among other conditions, attention deficit hyperactivity disorder. Here, we demonstrate that KLHL12, a BTB-Kelch protein, specifically binds to this polymorphic region of the D4 receptor through its Kelch domain. Moreover, we show that KLHL12 also interacts with Cullin3 and thereby functions as an adaptor to target the D4 receptor to an E3 ubiquitin ligase complex. By ubiquitination assays in eukaryotic cells, we further demonstrate that overexpression of KLHL12 strongly promotes ubiquitination of the D4 receptor. In addition, we show that also other dopamine receptor subtypes undergo basal ubiquitination, but this is not affected by KLHL12. These data are the first to show ubiquitination of dopamine receptors and the first to identify a protein specifically interacting with the D4 polymorphism, thereby building up an E3 ligase complex with substrate specificity toward the D4 receptor.     

Breast cancer metastasis suppressor 1 (BRMS1) is destabilized by the Cul3-SPOP E3 ubiquitin ligase complex

Breast cancer metastasis suppressor 1 (BRMS1) suppresses metastasis without affecting primary tumorigenesis. The regulatory mechanism of BRMS1 at the protein level has not been revealed until recently. Here, we found that cullin 3 (Cul3), a component of E3 ubiquitin ligase, is a new binding partner of BRMS1 and the interaction between BRMS1 and Cul3 is mediated by the SPOP adaptor protein. Intriguingly, BRMS1 turns out to be a potent substrate that is ubiquitinated by the Cul3–SPOP complex. Knockdown of SPOP increases the level of BRMS1 protein and represses the expression of BRMS1 repressive target genes such as OPN and uPA in breast cancer cells. These results suggest that the novel regulatory mechanism of BRMS1 by Cul3–SPOP complex is important for breast cancer progression.     

Kinase-independent function of cyclin E

E-type cyclins are thought to drive cell-cycle progression by activating cyclin-dependent kinases, primarily CDK2. We previously found that cyclin E-null cells failed to incorporate MCM helicase into DNA prereplication complex during G(0) --> S phase progression. We now report that a kinase-deficient cyclin E mutant can partially restore MCM loading and S phase entry in cyclin E-null cells. We found that cyclin E is loaded onto chromatin during G(0) --> S progression. In the absence of cyclin E, CDT1 is normally loaded onto chromatin, whereas MCM is not, indicating that cyclin E acts between CDT1 and MCM loading. We observed a physical association of cyclin E with CDT1 and with MCMs. We propose that cyclin E facilitates MCM loading in a kinase-independent fashion, through physical interaction with CDT1 and MCM. Our work indicates that-in addition to their function as CDK activators-E cyclins play kinase-independent functions in cell-cycle progression.     

The proto-oncogene Int6 is essential for neddylation of Cul1 and Cul3 in Drosophila

Int6 is a proto-oncogene implicated in various types of cancer, but the mechanisms underlying its activity are not clear. Int6 encodes a subunit of the eukaryotic translation initiation factor 3, and interacts with two related complexes, the proteasome, whose activity is regulated by Int6 in S. pombe, and the COP9 signalosome. The COP9 signalosome regulates the activity of Cullin-Ring Ubiquitin Ligases via deneddylation of their cullin subunit. We report here the generation and analysis of two Drosophila mutants in Int6. The mutants are lethal demonstrating that Int6 is an essential gene. The mutant larvae accumulate high levels of non-neddylated Cul1, suggesting that Int6 is a positive regulator of cullin neddylation. Overexpression in Int6 in cell culture leads to accumulation of neddylated cullins, further supporting a positive role for Int6 in regulating neddylation. Thus Int6 and the COP9 signalosome play opposing roles in regulation of cullin neddylation.     

IkappaB kinase alpha regulates subcellular distribution and turnover of cyclin D1 by phosphorylation

IkappaB kinases (IKKs), IKKalpha and IKKbeta, with a regulatory subunit IKKgamma/NEMO constitute a high molecular weight IKK complex that regulates NF-kappaB activity. Although IKKalpha and IKKbeta share structural and biochemical similarities, IKKalpha has been shown to have distinct biological roles. Here we show that IKKalpha plays a critical role in regulating cyclin D1 during the cell cycle. Analysis of IKKalpha-/- mouse embryo fibroblast cells showed that cyclin D1 is overexpressed and localized in the nucleus compared with parental mouse embryo fibroblasts. IKKalpha associates with and phosphorylates cyclin D1. Analysis on cyclin D1 mutants demonstrated that IKKalpha phosphorylates cyclin D1 at Thr286. Reconstitution of IKKalpha in knockout cells leads to nuclear export and increased degradation of cyclin D1. Further, RNAi-mediated knockdown of IKKalpha results in similar changes as observed in IKKalpha-/- cells. These results suggest a novel role of IKKalpha in regulating subcellular localization and proteolysis of cyclin D1 by phosphorylation of cyclin D1 at Thr286, the same residue earlier found to be a target for glycogen synthase kinase-3beta-induced phosphorylation.     

Cyclin E2 induces genomic instability by mechanisms distinct from cyclin E1

Cyclins E1 drives the initiation of DNA replication, and deregulation of its periodic expression leads to mitotic delay associated with genomic instability. Since it is not known whether the closely related protein cyclin E2 shares these properties, we overexpressed cyclin E2 in breast cancer cells. This did not affect the duration of mitosis, nor did it cause an increase in p107 association with CDK2. In contrast, cyclin E1 overexpression led to inhibition of the APC complex, prolonged metaphase and increased p107 association with CDK2. Despite these different effects on the cell cycle, elevated levels of either cyclin E1 or E2 led to hallmarks of genomic instability, i.e., an increased proportion of abnormal mitoses, micronuclei and chromosomal aberrations. Cyclin E2 induction of genomic instability by a mechanism distinct from cyclin E1 indicates that these two proteins have unique functions in a cancer setting.     

The tuberous sclerosis complex-mammalian target of rapamycin pathway maintains the quiescence and survival of naive T cells

Naive T cells receive stimulation from the positive selecting ligand in the periphery for their survival. This stimulation does not normally lead to overt activation of T cells, as the T cells remain largely quiescent until they receive either antigenic or lymphopenic stimuli. The underlying mechanism responsible for survival and quiescence of the naive T cells remains largely unknown. In this study, we report that T cell-specific deletion of Tsc1, a negative regulator of mammalian target of rapamycin, resulted in both spontaneous losses of quiescence and cellularity, especially within the CD8 subset. The Tsc1-deficient T cells have increased cell proliferation and apoptosis. Tsc1 deletion affects the survival and quiescence of T cells in the absence of antigenic stimulation. Loss of quiescence but not cellularity was inhibited by rapamycin. Our data demonstrate that tuberous sclerosis complex-mammalian target of rapamycin maintains quiescence and survival of T cells.