Thomas J. Hope

A leucine‐rich nuclear export signal in the p53 tetramerization domain: regulation of subcellular localization and p53 activity by NES masking

Appropriate subcellular localization is crucial for regulating p53 function. We show that p53 export is mediated by a highly conserved leucine‐rich nuclear export signal (NES) located in its tetramerization domain. Mutation of NES residues prevented p53 export and hampered tetramer formation. Although the p53‐binding protein MDM2 has an NES and has been proposed to mediate p53 export, we show that the intrinsic p53 NES is both necessary and sufficient for export. This report also demonstrates that the cytoplasmic localization of p53 in neuroblastoma cells is due to its hyperactive nuclear export: p53 in these cells can be trapped in the nucleus by the export‐inhibiting drug leptomycin B or by binding a p53‐tetramerization domain peptide that masks the NES. We propose a model in which regulated p53 tetramerization occludes its NES, thereby ensuring nuclear retention of the DNA‐binding form. We suggest that attenuation of p53 function involves the conversion of tetramers into monomers or dimers, in which the NES is exposed to the proteins which mediate their export to the cytoplasm.

Visualization of the intracellular behavior of HIV in living cells

To track the behavior of human immunodeficiency virus (HIV)-1 in the cytoplasm of infected cells, we have tagged virions by incorporation of HIV Vpr fused to the GFP. Observation of the GFP-labeled particles in living cells revealed that they moved in curvilinear paths in the cytoplasm and accumulated in the perinuclear region, often near the microtubule-organizing center. Further studies show that HIV uses cytoplasmic dynein and the microtubule network to migrate toward the nucleus. By combining GFP fused to the NH2 terminus of HIV-1 Vpr tagging with other labeling techniques, it was possible to determine the state of progression of individual particles through the viral life cycle. Correlation of immunofluorescent and electron micrographs allowed high resolution imaging of microtubule-associated structures that are proposed to be reverse transcription complexes. Based on these observations, we propose that HIV uses dynein and the microtubule network to facilitate the delivery of the viral genome to the nucleus of the cell during early postentry steps of the HIV life cycle.

ES cells do not activate p53-dependent stress responses and undergo p53-independent apoptosis in response to DNA damage

BACKGROUND Embryonic stem (ES) cells can contribute precursors to all adult cell lineages. Consequently, damage to ES cell genomes may cause serious developmental malfunctions. In somatic cells, cell-cycle checkpoints limit DNA damage by preventing DNA replication under conditions that may produce chromosomal aberrations. The tumor suppressor p53 is involved in such checkpoint controls and is also required to avoid a high rate of embryonic malformations. We characterized the cell-cycle and DNA-damage responses of ES cells to elucidate the mechanisms that prevent accumulation or transmission of damaged genomes during development. RESULTS ES cells derived from wild-type mice did not undergo cell-cycle arrest in response to DNA damage or nucleotide depletion, although they synthesized abundant quantities of p53. The p53 protein in ES cells was cytoplasmic and translocated inefficiently to the nucleus upon nucleotide depletion. Expression of high levels of active p53 from an adenovirus vector could not trigger cell cycle arrest. Instead, ES cells that sustained DNA damage underwent p53-independent apoptosis. The antimetabolite-induced p53-dependent arrest response was restored in ES cells upon differentiation. CONCLUSIONS Cell-cycle regulatory pathways in early embryos differ significantly from those in differentiated somatic cells. In undifferentiated ES cells, p53 checkpoint pathways are compromised by factors that affect the nuclear localization of p53 and by the loss of downstream factors that are necessary to induce cell-cycle arrest. A p53-independent programmed cell death pathway is effectively employed to prevent cells with damaged genomes from contributing to the developing organism. The p53-mediated checkpoint controls become important when differentiation occurs.

Polyreactivity increases the apparent affinity of anti-HIV antibodies by heteroligation

During immune responses, antibodies are selected for their ability to bind to foreign antigens with high affinity, in part by their ability to undergo homotypic bivalent binding. However, this type of binding is not always possible. For example, the small number of gp140 glycoprotein spikes displayed on the surface of the human immunodeficiency virus (HIV) disfavours homotypic bivalent antibody binding. Here we show that during the human antibody response to HIV, somatic mutations that increase antibody affinity also increase breadth and neutralizing potency. Surprisingly, the responding naive and memory B cells produce polyreactive antibodies, which are capable of bivalent heteroligation between one high-affinity anti-HIV-gp140 combining site and a second low-affinity site on another molecular structure on HIV. Although cross-reactivity to self-antigens or polyreactivity is strongly selected against during B-cell development, it is a common serologic feature of certain infections in humans, including HIV, Epstein-Barr virus and hepatitis C virus. Seventy-five per cent of the 134 monoclonal anti-HIV-gp140 antibodies cloned from six patients with high titres of neutralizing antibodies are polyreactive. Despite the low affinity of the polyreactive combining site, heteroligation demonstrably increases the apparent affinity of polyreactive antibodies to HIV.

Actin is part of pre-initiation complexes and is necessary for transcription by RNA polymerase II

Actin is abundant in the nucleus and has been implicated in transcription; however, the nature of this involvement has not been established. Here we demonstrate that β-actin is critically involved in transcription because antibodies directed against β-actin, but not muscle actin, inhibited transcription in vivo and in vitro. Chromatin immunoprecipitation assays demonstrated the recruitment of actin to the promoter region of the interferon-γ-inducible MHC2TA gene as well as the interferon-α-inducible G1P3 gene. Further investigation revealed that actin and RNA polymerase II co-localize in vivo and also co-purify. We employed an in vitro system with purified nuclear components to demonstrate that antibodies to β-actin block the initiation of transcription. This assay also demonstrates that β-actin stimulates transcription by RNA polymerase II. Finally, DNA-binding experiments established the presence of β-actin in pre-initiation complexes and also showed that the depletion of actin prevented the formation of pre-initiation complexes. Together, these data suggest a fundamental role for actin in the initiation of transcription by RNA polymerase II.

PRC1: A Human Mitotic Spindle–Associated CDK Substrate Protein Required for Cytokinesis

We have identified a novel human protein, PRC1, that is involved in cytokinesis. PRC1 is a good substrate for several CDKs in vitro and is phosphorylated in vivo at sites that are phosphorylated by CDK in vitro, strongly suggesting that PRC1 is an in vivo CDK substrate. PRC1 has sequence homology to the budding yeast anaphase spindle elongation factor Ase1p. Like Ase1p, PRC1 protein levels are high during S and G2/M and drop dramatically after cells exit mitosis and enter G1. PRC1 is a nuclear protein in interphase, becomes associated with mitotic spindles in a highly dynamic manner during mitosis, and localizes to the cell mid-body during cytokinesis. Microinjection of anti-PRC1 antibodies into HeLa cells blocked cellular cleavage, but not nuclear division, indicating a functional role for PRC1 in the process of cytokinesis.

An N-terminal nuclear export signal is required for the nucleocytoplasmic shuttling of IκBα

The potent transcriptional activities of Rel/NF‐κB proteins are regulated in the cytoplasm and nucleus by the inhibitor, IκBα. The mechanism, by which IκBα can either sequester NF‐κB in the cytoplasm or act as a nuclear post‐induction repressor of NF‐κB, is uncertain. We find that IκBα shuttles continuously between the nucleus and cytoplasm. This shuttling requires a previously unidentified CRM1‐dependent nuclear export signal (NES) located within the N‐terminal domain of IκBα at amino acids 45–55. Deletion or mutation of the N‐terminal NES results in nuclear localization of IκBα. NF‐κB (p65) association with IκBα affects steady‐state localization but does not inhibit its shuttling. Endogenous complexes of IκBα–NF‐κB shuttle and will accumulate in the nucleus when CRM1 export is blocked. We find TNFα can activate the nuclear IκBα–NF‐κB complexes by the classical mechanism of proteasome‐mediated degradation of IκBα. These studies reveal a more dynamic nucleocytoplasmic distribution for IκBα and NF‐κB suggesting previously unknown strategies for regulating this ubiquitous family of transcription activators.

Enhanced Expression of Transgenes from Adeno-Associated Virus Vectors with the Woodchuck Hepatitis Virus Posttranscriptional Regulatory Element: Implications for Gene Therapy

The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) evolved to stimulate the expression of intronless viral messages. To determine whether this ability to enhance expression could be useful in nonviral and heterologous viral gene delivery systems, we analyzed the ability of the WPRE to elevate the expression of a cDNA encoding the green fluorescent protein (GFP) in these contexts. We find that the WPRE can stimulate the expression of GFP when the gene is delivered by transfection or transduction with recombinant adeno-associated virus (AAV). Enhancement occurred both during transient expression and when the gene is stably incorporated into the genome of target cells. This enhancement required that the WPRE be located in cis within the GFP message, and was observed in both transformed cell lines and primary human fibroblasts. These results demonstrate that the WPRE will be an effective tool for increasing the long-term expression of transgenes in gene therapy.

Human Immunodeficiency Virus Type 1 Is Trapped by Acidic but Not by Neutralized Human Cervicovaginal Mucus

ABSTRACT To reliably infect a primate model for human immunodeficiency virus (HIV), ∼10,000-fold more virus must be delivered vaginally than intravenously. However, the vaginal mechanisms that help protect against HIV are poorly understood. Here, we report that human cervicovaginal mucus (CVM), obtained from donors with normal lactobacillus-dominated vaginal flora, efficiently traps HIV, causing it to diffuse more than 1,000-fold more slowly than it does in water. Lactobacilli acidify CVM to pH ∼4 by continuously producing lactic acid. At this acidic pH, we found that lactic acid, but not HCl, abolished the negative surface charge on HIV without lysing the virus membrane. In contrast, in CVM neutralized to pH 6 to 7, as occurs when semen temporarily neutralizes the vagina, HIV maintained its native surface charge and diffused only 15-fold more slowly than it would in water. Thus, methods that can maintain both a high lactic acid content and acidity for CVM during coitus may contribute to both vaginal and penile protection by trapping HIV before it can reach target cells. Our results reveal that CVM likely plays an important but currently unappreciated role in decreasing the rate of HIV sexual transmission.

Mammalian Cdc7–Dbf4 protein kinase complex is essential for initiation of DNA replication

The Cdc7–Dbf4 kinase is essential for regulating initiation of DNA replication in Saccharomyces cerevisiae. Previously, we identified a human Cdc7 homolog, HsCdc7. In this study, we report the identification of a human Dbf4 homolog, HsDbf4. We show that HsDbf4 binds to HsCdc7 and activates HsCdc7 kinase activity when HsDbf4 and HsCdc7 are coexpressed in insect and mammalian cells. HsDbf4 protein levels are regulated during the cell cycle with a pattern that matches that of HsCdc7 protein kinase activity. They are low in G1, increase during G1–S, and remain high during S and G2–M. Purified baculovirus‐expressed HsCdc7–HsDbf4 selectively phosphorylates the MCM2 subunit of the minichromosome maintenance (MCM) protein complex isolated by immunoprecipitation with MCM7 antibodies in vitro. Two‐dimensional tryptic phosphopeptide‐mapping analysis of in vivo 32P‐labeled MCM2 from HeLa cells reveals that several major tryptic phosphopeptides of MCM2 comigrate with those of MCM2 phosphorylated by HsCdc7–HsDbf4 in vitro, suggesting that MCM2 is a physiological HsCdc7–HsDbf4 substrate. Immunoneutralization of HsCdc7–HsDbf4 activity by microinjection of anti‐HsCdc7 antibodies into HeLa cells blocks initiation of DNA replication. These results indicate that the HsCdc7–HsDbf4 kinase is directly involved in regulating the initiation of DNA replication by targeting MCM2 protein in mammalian cells.