A complex degradation signal in Cyclin A required for G1 arrest, and a C-terminal region for mitosis

The destruction box (D-box) consensus sequence has been defined as a motif mediating polyubiquitylation and proteolysis of B-type cyclins during mitosis. We show here that the regions with similarity to D-boxes are not required for mitotic degradation of Drosophila Cyclin A. Instead of a simple D-box, a complex N-terminal degradation signal is present in this cyclin. Mutations that impair or abolish mitotic Cyclin A destruction delay progression through metaphase, but only when overexpressed. Moreover, these mutations prevent epidermal cells from entering the first G1 phase of embryogenesis and lead to a complete extra division cycle instead of a timely cell proliferation arrest. Residual Cyclin A activity after mitosis, therefore, has S phase-promoting activity. In principle, an S phase defect could also explain why epidermal cells fail to enter mitosis 16 in mutants lacking zygotic Cyclin A function. However, we demonstrate that this failure of mitosis is not caused simply by DNA replication or damage checkpoints. Entry into mitosis requires a function of Cyclin A that does not depend on the presence of the N-terminal region.     

Control the Host Cell Cycle: Viral Regulation of the Anaphase-Promoting Complex

Viruses commonly manipulate cell cycle progression to create cellular conditions that are most beneficial to their replication. To accomplish this feat, viruses often target critical cell cycle regulators in order to have maximal effect with minimal input. One such master regulator is the large, multisubunit E3 ubiquitin ligase anaphase-promoting complex (APC) that targets effector proteins for ubiquitination and proteasome degradation. The APC is essential for cells to progress through anaphase, exit from mitosis, and prevent a premature entry into S phase. These far-reaching effects of the APC on the cell cycle are through its ability to target a number of substrates, including securin, cyclin A, cyclin B, thymidine kinase, geminin, and many others. Recent studies have identified several proteins from a number of viruses that can modulate APC activity by different mechanisms, highlighting the potential of the APC in driving viral replication or pathogenesis. Most notably, human cytomegalovirus (HCMV) protein pUL21a was recently identified to disable the APC via a novel mechanism by targeting APC subunits for degradation, both during virus infection and in isolation. Importantly, HCMV lacking both viral APC regulators is significantly attenuated, demonstrating the impact of the APC on a virus infection. Work in this field will likely lead to novel insights into viral replication and pathogenesis and APC function and identify novel antiviral and anticancer targets. Here we review viral mechanisms to regulate the APC, speculate on their roles during infection, and identify questions to be addressed in future studies.     

DNA damage leads to a Cyclin A-dependent delay in metaphase-anaphase transition in the Drosophila gastrula

BACKGROUND: In response to DNA damage, fission yeast, mammalian cells, and cells of the Drosophila gastrula inhibit Cdk1 to delay the entry into mitosis. In contrast, budding yeast delays metaphase-anaphase transition by stabilization of an anaphase inhibitor, Pds1p. A variation of the second response is seen in Drosophila cleavage embryos; when nuclei enter mitosis with damaged DNA, centrosomes lose gamma-tubulin, spindles lose astral microtubules, chromosomes fail to reach a metaphase configuration, and interphase resumes without an intervening anaphase. The resulting polyploid nuclei are eliminated. RESULTS: The cells of the Drosophila gastrula can also delay metaphase-anaphase transition in response to DNA damage. This delay accompanies the stabilization of Cyclin A, a known inhibitor of sister chromosome separation in Drosophila. Unlike in cleavage embryos, gamma-tubulin remains at the spindle poles, and anaphase always occurs after the delay. Cyclin A mutants fail to delay metaphase-anaphase transition after irradiation and show an increased frequency of chromosome breakage in the subsequent anaphase. CONCLUSIONS: DNA damage delays metaphase-anaphase transition in Drosophila by stabilizing Cyclin A. This delay may normally serve to preserve chromosomal integrity during segregation. To our knowledge this is the first report of a metazoan metaphase-anaphase transition being delayed in response to DNA damage. Though mitotic progression is modulated in response to DNA damage in both cleaving and gastruating embryos of Drosophila, different mechanisms operate. These differences are discussed in the context of differential cell cycle regulation in cleavage and gastrula stages.     

Cyclin B1 is rate limiting but not essential for mitotic entry and progression in mammalian somatic cells

Cyclin B1 should have some rate limiting function for cell cycle progression. To test this, we measured the effect of siRNA-mediated depletion of cyclin B1 on mitotic entry and timing. We depleted cyclin B1 in HeLa and hTert-RPE1 cells to levels equivalent or below those achieved in the telophase-to-G1 window. Average cyclin B1/Cdk1 activity was measured in HeLa cells and depleted by ~99%. In both cell lines, this caused ~20% increase in the G2 and ~20% increase the M traverse time. However, co-depletion of cyclin B1 and B2 induced a profound increase in G2 cells, a dramatic reduction in mitotic cells, and an increase in a 4C cycling population. We conclude that any residual levels of cyclin B1 were not sufficient to promote stable mitotic entry and transition in absence of normal levels of cyclin B2. Therefore, we conclude that B cyclin is necessary for mitosis but cyclin B1 is not. Nocodazole treated, cyclin B1-depleted HeLa cells arrested but exited that arrest at higher rates than controls, suggesting that the duration of the spindle checkpoint was affected. In B1 depleted cells, population growth was delayed but evidence of cell death was not consistently observed. A strong phenotype of mitotic chromosomal aberration was observed in HeLa cells depleted for either cyclin but not in RPE cells. In B1 or B2 depleted cells, maloriented chromosomes at metaphase were increased 10 fold and one third of affected metaphase cells entered anaphase without congression. Lagging chromosomes at anaphase were dramatically increased. The aggregate evidence from our study and others suggests that the common effect of cyclin B1 depletion is mild cell cycle perturbation. Lack of uniformity in other phenotypes suggest that these are low penetrance effects that are exacerbated or compensated in some systems by other mechanisms.     

Cyclin A Degradation by Primate Cytomegalovirus Protein pUL21a Counters Its Innate Restriction of Virus Replication

Cyclin A is critical for cellular DNA synthesis and S phase progression of the cell cycle. Human cytomegalovirus (HCMV) can reduce cyclin A levels and block cellular DNA synthesis, and cyclin A overexpression can repress HCMV replication. This interaction has only been previously observed in HCMV as murine CMV does not downregulate cyclin A, and the responsible viral factor has not been identified. We previously reported that the HCMV protein pUL21a disrupted the anaphase-promoting complex (APC), but a point mutant abrogating this activity did not phenocopy a UL21a-deficient virus, suggesting that pUL21a has an additional function. Here we identified a conserved arginine-x-leucine (RxL) cyclin-binding domain within pUL21a, which allowed pUL21a to interact with cyclin A and target it for proteasome degradation. Homologous pUL21a proteins from both chimpanzee and rhesus CMVs also contained the RxL domain and similarly degraded cyclin A, indicating that this function is conserved in primate CMVs. The RxL point mutation disabled the virus'' ability to block cellular DNA synthesis and resulted in a growth defect similar to pUL21a-deficient virus. Importantly, knockdown of cyclin A rescued growth of UL21a-deficient virus. Together, these data show that during evolution, the pUL21a family proteins of primate CMVs have acquired a cyclin-binding domain that targets cyclin A for degradation, thus neutralizing its restriction on virus replication. Finally, the combined proteasome-dependent degradation of pUL21a and its cellular targets suggests that pUL21a may act as a novel suicide protein, targeting its protein cargos for destruction.     

Cell cycle-independent expression of immediate-early gene 3 results in G1 and G2 arrest in murine cytomegalovirus-infected cells

The infectious cycle of human cytomegalovirus (HCMV) is intricately linked to the host's cell cycle. Viral gene expression can be initiated only in G₀/G₁ phase. Once expressed, the immediate-early gene product IE2 prevents cellular DNA synthesis, arresting infected cells with a G₁ DNA content. This function is required for efficient viral replication in vitro. A prerequisite for addressing its in vivo relevance is the characterization of cell cycle-regulatory activities of CMV species for which animal models have been established. Here, we show that murine CMV (MCMV), like HCMV, has a strong antiproliferative capacity and arrests cells in G₁. Unexpectedly, and in contrast to HCMV, MCMV can also block cells that have passed through S phase by arresting them in G₂. Moreover, MCMV can also replicate in G₂ cells. This is made possible by the cell cycle-independent expression of MCMV immediate-early genes. Transfection experiments show that of several MCMV candidate genes, only immediate-early gene 3 (ie3), the homologue of HCMV IE2, exhibits cell cycle arrest activity. Accordingly, an MCMV ie3 deletion mutant has lost the ability to arrest cells in either G₁ or G₂. Thus, despite interspecies variations in the cell cycle dependence of viral gene expression, the central theme of HCMV IE2-induced cell cycle arrest is conserved in the murine counterpart, raising the possibility of studying its physiological relevance at the level of the whole organism.     

Parvovirus-Induced Depletion of Cyclin B1 Prevents Mitotic Entry of Infected Cells

Parvoviruses halt cell cycle progression following initiation of their replication during S-phase and continue to replicate their genomes for extended periods of time in arrested cells. The parvovirus minute virus of mice (MVM) induces a DNA damage response that is required for viral replication and induction of the S/G2 cell cycle block. However, p21 and Chk1, major effectors typically associated with S-phase and G2-phase cell cycle arrest in response to diverse DNA damage stimuli, are either down-regulated, or inactivated, respectively, during MVM infection. This suggested that parvoviruses can modulate cell cycle progression by another mechanism. In this work we show that the MVM-induced, p21- and Chk1-independent, cell cycle block proceeds via a two-step process unlike that seen in response to other DNA-damaging agents or virus infections. MVM infection induced Chk2 activation early in infection which led to a transient S-phase block associated with proteasome-mediated CDC25A degradation. This step was necessary for efficient viral replication; however, Chk2 activation and CDC25A loss were not sufficient to keep infected cells in the sustained G2-arrested state which characterizes this infection. Rather, although the phosphorylation of CDK1 that normally inhibits entry into mitosis was lost, the MVM induced DDR resulted first in a targeted mis-localization and then significant depletion of cyclin B1, thus directly inhibiting cyclin B1-CDK1 complex function and preventing mitotic entry. MVM infection thus uses a novel strategy to ensure a pseudo S-phase, pre-mitotic, nuclear environment for sustained viral replication.     

The human cytomegalovirus IE86 protein can block cell cycle progression after inducing transition into the S phase of permissive cells

Human cytomegalovirus (HCMV) infection of permissive cells has been reported to induce a cell cycle halt. One or more viral proteins may be involved in halting progression at different stages of the cell cycle. We investigated how HCMV infection, and specifically IE86 protein expression, affects the cell cycles of permissive and nonpermissive cells. We used a recombinant virus that expresses the green fluorescent protein (GFP) to determine the effects of HCMV on the cell cycle of permissive cells. Fluorescence by GFP allowed us to select for only productively infected cells. Replication-defective adenovirus vectors expressing the IE72 or IE86 protein were also used to efficiently transduce 95% or more of the cells. The adenovirus-expressed IE86 protein was determined to be functional by demonstrating negative autoregulation of the major immediate-early promoter and activation of an early viral promoter in the context of the viral genome. To eliminate adenovirus protein effects, plasmids expressing GFP for fluorescent selection of only transfected cells and wild-type IE86 protein or a mutant IE86 protein were tested in permissive and nonpermissive cells. HCMV infection induced the entry of U373 cells into the S phase. All permissive cells infected with HCMV were blocked in cell cycle progression and could not divide. After either transduction or transfection and IE86 protein expression, the number of all permissive or nonpermissive cell types in the S phase increased significantly, but the cells could no longer divide. The IE72 protein did not have a significant effect on the S phase. Since IE86 protein inhibits cell cycle progression, the IE2 gene in a human fibroblast IE86 protein-expressing cell line was sequenced. The IE86 protein in these retrovirus-transduced cells has mutations in a critical region of the viral protein. The locations of the mutations and the function of the IE86 protein in controlling cell cycle progression are discussed.     

G1 tetraploidy checkpoint and the suppression of tumorigenesis

Checkpoints suppress improper cell cycle progression to ensure that cells maintain the integrity of their genome. During mitosis, a metaphase checkpoint requires the integration of all chromosomes into a metaphase array in the mitotic spindle prior to mitotic exit. Still, mitotic errors occur in mammalian cells with a relatively high frequency. Metaphase represents the last point of control in mitosis. Once the cell commits to anaphase there are no checkpoints to sense segregation defects. In this context, we will explore our recent finding that non-transformed mammalian cells have a checkpoint that acts subsequent to mitotic errors to block the proliferation of cells that have entered G1 with tetraploid status. This arrest is dependent upon both p53 and pRb, and may represent an important function of both p53 and pRb as tumor suppressors. Further, we discuss the possibility that this mechanism may similarly impose G1 arrest in cells that become aneuploid through errors in mitosis.     

Exit from mitosis is regulated by Drosophila fizzy and the sequential destruction of cyclins A, B and B3.

While entry into mitosis is triggered by activation of cdc2 kinase, exit from mitosis requires inactivation of this kinase. Inactivation results from proteolytic degradation of the regulatory cyclin subunits during mitosis. At least three different cyclin types, cyclins A, B and B3, associate with cdc2 kinase in higher eukaryotes and are sequentially degraded in mitosis. We show here that mutations in the Drosophila gene fizzy (fzy) block the mitotic degradation of these cyclins. Moreover, expression of mutant cyclins (delta cyclins) lacking the destruction box motif required for mitotic degradation affects mitotic progression at distinct stages. Deltacyclin A results in a delay in metaphase, deltacyclin B in an early anaphase arrest and deltacyclin B3 in a late anaphase arrest, suggesting that mitotic progression beyond metaphase is ordered by the sequential degradation of these different cyclins. Coexpression of deltacyclins A, B and B3 allows a delayed separation of sister chromosomes, but interferes wit chromosome segregation to the poles. Mutations in fzy block both sister chromosome separation and segregation, indicating that fzy plays a crucial role in the metaphase/anaphase transition.     

The US16 gene of human cytomegalovirus is required for efficient viral infection of endothelial and epithelial cells

The human cytomegalovirus (HCMV) US12 gene family comprises a set of 10 contiguous genes (US12 to US21), each encoding a predicted seven-transmembrane protein and whose specific functions have yet to be ascertained. While inactivation of individual US12 family members in laboratory strains of HCMV has not been found to affect viral replication in fibroblasts, inactivation of US16 was reported to increase replication in microvascular endothelial cells. Here, we investigate the properties of US16 further by ascertaining the expression pattern of its product. A recombinant HCMV encoding a tagged version of the US16 protein expressed a 33-kDa polypeptide that accumulated with late kinetics in the cytoplasmic virion assembly compartment. To elucidate the function(s) of pUS16, we generated US16-deficient mutants in the TR clinical strain of HCMV. According to previous studies, inactivation of US16 had no effect on viral replication in fibroblasts. In contrast, the US16-deficient viruses exhibited a major growth defect in both microvascular endothelial cells and retinal pigment epithelial cells. The expression of representative IE, E, and L viral proteins was impaired in endothelial cells infected with a US16 mutant virus, suggesting a defect in the replication cycle that occurs prior to IE gene expression. This defect must be due to an inefficient entry and/or postentry event, since pp65 and viral DNA did not move to the nucleus in US16 mutant-infected cells. Taken together, these data indicate that the US16 gene encodes a novel virus tropism factor that regulates, in a cell-specific manner, a pre-immediate-early phase of the HCMV replication cycle.     

Targeting of the gene product encoded by ORF UL56 of human cytomegalovirus into viral replication centers

The highly conserved DNA-binding protein pUL56 of human cytomegalovirus (HCMV) was found to be predominantly localized throughout the nucleus as well as in viral replication centers of infected cells. The latter localization was abolished by phosphono acetic acid, an inhibitor of viral DNA replication. Immunofluorescence revealed that pUL56 co-localized in replication centers alongside pUL112-113 and pUL44 at late times of infection. By co-immunoprecipitations, a direct interaction with pUL44, a protein of the replication fork, was detected. These results showed for the first time that HCMV pUL56 is localized in viral replication centers, implicating that DNA replication is coupled with packaging.     

mei-41 and bub1 block mitosis at two distinct steps in response to incomplete DNA replication in Drosophila embryos.

Drosophila double park encodes a homolog of Cdt1 that functions in initiation of DNA replication in fission yeast and Xenopus. dup mutants complete the first 15 embryonic cell cycles, presumably via maternal dup products, and show defects in the 16(th) S phase (S16). Cells carrying dup(a1) allele forgo S16 altogether but enter mitosis 16 (M16). We find that the timing of entry into M16 is similar in dup(a1) and heterozygous or wild-type (wt) controls. In contrast, we find that mutant cells carrying another allele, dup(a3), undergo a partial S16 and delay the entry into M16. Thus, initiation of S16 appears necessary for delaying M16. This delay is absent in double mutants of dup(a3) and mei-41 (Drosophila ATR), indicating that a mei-41-dependent checkpoint acts to delay the entry into mitosis in response to incomplete DNA replication. dup(a3) and dup(a1) mutant cells that enter M16 become arrested in M16. We find that mitotic cyclins are stabilized and that a spindle checkpoint protein, Bub1, localizes onto chromosomes during mitotic arrest in dup mutants. These features suggest an arrest prior to metaphase-anaphase transition. dup(a3) bub1 double mutant cells exit M16, indicating that a bub1-mediated checkpoint acts to block mitotic exit in dup mutants. To our knowledge, this is the first report of (1) incomplete DNA replication affecting both the entry into and the exit from mitosis in a single cell cycle via different mechanisms and (2) the role of bub1 in regulating mitotic exit in response to incomplete DNA replication.     

Human cytomegalovirus labeled with green fluorescent protein for live analysis of intracellular particle movements

Human cytomegalovirus (HCMV) replicates in the nuclei of infected cells. Successful replication therefore depends on particle movements between the cell cortex and nucleus during entry and egress. To visualize HCMV particles in living cells, we have generated a recombinant HCMV expressing enhanced green fluorescent protein (EGFP) fused to the C terminus of the capsid-associated tegument protein pUL32 (pp150). The resulting UL32-EGFP-HCMV was analyzed by immunofluorescence, electron microscopy, immunoblotting, confocal microscopy, and time-lapse microscopy to evaluate the growth properties of this virus and the dynamics of particle movements. UL32-EGFP-HCMV replicated similarly to wild-type virus in fibroblast cultures. Green fluorescent virus particles were released from infected cells. The fluorescence stayed associated with particles during viral entry, and fluorescent progeny particles appeared in the nucleus at 44 h after infection. Surprisingly, strict colocalization of pUL32 and the major capsid protein pUL86 within nuclear inclusions indicated that incorporation of pUL32 into nascent HCMV particles occurred simultaneously with or immediately after assembly of the capsid. A slow transport of nuclear particles towards the nuclear margin was demonstrated. Within the cytoplasm, most particles performed irregular short-distance movements, while a smaller fraction of particles performed centripetal and centrifugal long-distance movements. Although numerous particles accumulated in the cytoplasm, release of particles from infected cells was a rare event, consistent with a release rate of about 1 infectious unit per h per cell in HCMV-infected fibroblasts as calculated from single-step growth curves. UL32-EGFP-HCMV will be useful for further investigations into the entry, maturation, and release of this virus.