Uracil- and Pyridine-Containing HDAC Inhibitors Displayed Cytotoxicity in Colorectal and Glioblastoma Cancer Stem Cells

Cancer stem cells (CSCs) are a niche of highly tumorigenic cells featuring self-renewal, activation of pluripotency genes, multidrug resistance, and ability to cause cancer relapse. Seven HDACi (1-7), showing either hydroxamate or 2'-aminoanilide function, were tested in colorectal cancer (CRC) and glioblastoma multiforme (GBM) CSCs to determine their effects on cell proliferation, H3 acetylation levels and in-cell HDAC activity. Two uracil-based hydroxamates, 5 and 6, which differ in substitution at C5 and C6 positions of the pyrimidine ring, exhibited the greatest cytotoxicity in GBM (5) and CRC (6) CSCs, followed by the pyridine-hydroxamate 2, with 2- to 6-fold higher potency than the positive control SAHA. Finally, increased H3 acetylation as well as HDAC inhibition directly in cells by selected 2'-aminoanilide 4 and hydroxamate 5 confirmed target engagement. Further investigation will be conducted into the broad-spectrum anticancer properties of the most potent derivatives and their effects in combination with approved, conventional anticancer drugs.

Safety and effectiveness of a somatropin biosimilar in children requiring growth hormone treatment: second analysis of the PATRO Children study Italian cohort

PURPOSE

To investigate the long-term safety (primary endpoint) and effectiveness (secondary endpoint) of the somatropin biosimilar Omnitrope^®.

METHODS

PATRO Children is an ongoing, multicenter, observational, post-marketing surveillance study. Children who received Omnitrope^® for any indication were included. Adverse events (AEs) were evaluated in all study participants. Auxological data, including height standard deviation scores (HSDS) and height velocity standard deviation scores (HVSDS), were used to assess effectiveness. In this snapshot analysis, data from the Italian subpopulation up to August 2017 were reported.

RESULTS

A total of 291 patients (mean age 10.0 years, 56.0% male) were enrolled at 19 sites in Italy. The mean duration of Omnitrope^® treatment was 33.1 ± 21.7 months. There were 48 AEs with a suspected relationship to the study drug (as reported by the investigator) that occurred in 35 (12.0%) patients, most commonly headache, pyrexia, arthralgia, insulin-like growth factor above normal range, abdominal pain, pain in extremity and acute gastroenteritis. There were no confirmed cases of type 1 or type 2 diabetes; however, two patients (0.7%) had impaired glucose tolerance that was considered Omnitrope^® related. The mean HSDS increased from - 2.41 ± 0.73 at baseline (n = 238) to - 0.91 ± 0.68 at 6.5 years (n = 10). The mean HVSDS increased from - 1.77 ± 1.38 at baseline (n = 136) to 0.96 ± 1.13 at 6.5 years (n = 10).

CONCLUSIONS

In this sub-analysis of PATRO Children, Omnitrope^® appeared to have acceptable safety and effectiveness in the treatment of in Italian children, which was consistent with the earlier findings from controlled clinical trials.

Safety and effectiveness of Omnitrope® in patients with growth hormone deficiency: snapshot analysis of PATRO Adults study in the Italian population

PURPOSE

PATRO adults is an ongoing, multicenter, observational, post-marketing surveillance study aimed at investigating the long-term safety (primary endpoint) and effectiveness (secondary endpoint) of the recombinant human growth hormone (rhGH) Omnitrope® during routine clinical practice. This report describes data from Italian participants in PATRO Adults with growth hormone deficiency (GHD), up to August 2017.

METHODS

Participants were adults (aged > 18 years) with GHD requiring rhGH therapy and were prescribed Omnitrope®, including those who had previously received another rhGH product. Adverse events (AEs) were evaluated in all study participants. Data were collected on insulin-like growth factor (IGF)-I levels and cardiovascular risk factors, including blood pressure, lipids, and anthropometric parameters.

RESULTS

From September 2007 to August 2017, 88 patients (mean age 48.9 years, 58.0% male) were enrolled at 8 sites in Italy. The mean treatment duration with Omnitrope® was 51.5 ± 37 months. AEs occurred in 54 patients; the most common were asthenia (20.5%), headache (14.8%), and arthralgia (13.6%). Serious AEs occurred in 22 patients (25%), including pneumonia (n = 2) and renal failure (n = 2). Neoplasms (2 benign and 1 malignant) developed in three patients, but none were considered to be drug-related. There were no significant changes in fasting glucose or glycosylated hemoglobin (HbA1c) during the study period. Long-term Omnitrope® therapy showed slight positive effects on lipid profile, while no significant changes were observed in body weight and BMI during the study.

CONCLUSION

This snapshot analysis of Italian participants in PATRO Adults confirmed the long-term safety and effectiveness of Omnitrope® in adults with GHD.

Conjugates of Cryptophycin and RGD or isoDGR Peptidomimetics for Targeted Drug Delivery

RGD-cryptophycin and isoDGR-cryptophycin conjugates were synthetized by combining peptidomimetic integrin ligands and cryptophycin, a highly potent tubulin-binding antimitotic agent across lysosomally cleavable Val-Ala or uncleavable linkers. The conjugates were able to effectively inhibit binding of biotinylated vitronectin to integrin αvβ3, showing a binding affinity in the same range as that of the free ligands. The antiproliferative activity of the novel conjugates was evaluated on human melanoma cells M21 and M21-L with different expression levels of integrin αvβ3, showing nanomolar potency of all four compounds against both cell lines. Conjugates containing uncleavable linker show reduced activity compared to the corresponding cleavable conjugates, indicating efficient intracellular drug release in the case of cryptophycin-based SMDCs. However, no significant correlation between the in vitro biological activity of the conjugates and the integrin αvβ3 expression level was observed, which is presumably due to a non-integrin-mediated uptake. This reveals the complexity of effective and selective αvβ3 integrin-mediated drug delivery.

Synthesis and Biological Evaluation of RGD⁻Cryptophycin Conjugates for Targeted Drug Delivery

Cryptophycins are potent tubulin polymerization inhibitors with picomolar antiproliferative potency in vitro and activity against multidrug-resistant (MDR) cancer cells. Because of neurotoxic side effects and limited efficacy in vivo, cryptophycin-52 failed as a clinical candidate in cancer treatment. However, this class of compounds has emerged as attractive payloads for tumor-targeting applications. In this study, cryptophycin was conjugated to the cyclopeptide c(RGDfK), targeting integrin αvβ₃, across the protease-cleavable Val-Cit linker and two different self-immolative spacers. Plasma metabolic stability studies in vitro showed that our selected payload displays an improved stability compared to the parent compound, while the stability of the conjugates is strongly influenced by the self-immolative moiety. Cathepsin B cleavage assays revealed that modifications in the linker lead to different drug release profiles. Antiproliferative effects of Arg-Gly-Asp (RGD)⁻cryptophycin conjugates were evaluated on M21 and M21-L human melanoma cell lines. The low nanomolar in vitro activity of the novel conjugates was associated with inferior selectivity for cell lines with different integrin αvβ₃ expression levels. To elucidate the drug delivery process, cryptophycin was replaced by an infrared dye and the obtained conjugates were studied by confocal microscopy.

Blocking the association of HDAC4 with MAP1S accelerates autophagy clearance of mutant Huntingtin

Autophagy controls and executes the turnover of abnormally aggregated proteins. MAP1S interacts with the autophagy marker LC3 and positively regulates autophagy flux. HDAC4 associates with the aggregation-prone mutant huntingtin protein (mHTT) that causes Huntington's disease, and colocalizes with it in cytosolic inclusions. It was suggested HDAC4 interacts with MAP1S in a yeast two-hybrid screening. Here, we found that MAP1S interacts with HDAC4 via a HDAC4-binding domain (HBD). HDAC4 destabilizes MAP1S, suppresses autophagy flux and promotes the accumulation of mHTT aggregates. This occurs by an increase in the deacetylation of the acetylated MAP1S. Either suppression of HDAC4 with siRNA or overexpression of the MAP1S HBD leads to stabilization of MAP1S, activation of autophagy flux and clearance of mHTT aggregates. Therefore, specific interruption of the HDAC4-MAP1S interaction with short peptides or small molecules to enhance autophagy flux may relieve the toxicity of mHTT associated with Huntington's disease and improve symptoms of HD patients.

Smoothened antagonists: a promising new class of antitumor agents

BACKGROUND

Hedgehog signaling is essential for the development of most metazoans. In recent years, evidence has accumulated showing that many human tumors aberrantly re-activate this developmental signaling pathway and that interfering with it may provide a new strategy for the development of novel anti-cancer therapeutics. Smoothened is a G-protein coupled receptor-like protein that is essentially involved in hedgehog signal transduction and small molecule antagonists of Smoothened have started to show antitumor activity in preclinical models and in clinical trials.

OBJECTIVE

We critically review the role of hedgehog signaling in normal development and in human malignancies, the available drug discovery tools and the classes of small molecule inhibitors that are in development. We further aim to address the potential impact that pathway antagonists may have on the treatment options of cancer patients.

METHODS

Literature, patents and clinical trial results from the past 5 years were analyzed.

CONCLUSIONS

1) A large body of evidence suggests a frequent reactivation of hedgehog signaling in human cancer. 2) Smoothened is an attractive, highly druggable target with extensive preclinical and initial clinical validation in basal cell carcinoma. Several promising novel classes of Smoothened antagonists have been discovered and are being developed as anticancer agents. 3) Our knowledge of the biology of hedgehog signaling in cancer is still very incomplete and significant efforts will be required to understand how to use the emerging novel agents in the clinic.

Skin equivalents: a tool for the discovery and validation of pharmacodynamic biomarkers

In the development of targeted oncology drugs, it is important to assess drug effectiveness in individual patients. We evaluated the possibility of reproducing in an ex-vivo system the biological effects observed in vitro and in vivo by the combined administration of two chemotherapeutic drugs, gemcitabine and a small inhibitor of Wee1. We found that modulation of both CDC2 phosphorylation and of a previously-identified gene signature was detectable in human skin equivalents obtained with primary keratinocytes from three individuals. Therefore, we suggest that human skin equivalents could represent a promising tool for the identification and validation of novel pharmacodynamic biomarkers.

Discovery of salermide-related sirtuin inhibitors: binding mode studies and antiproliferative effects in cancer cells including cancer stem cells

Chemical changes performed on 1a (sirtinol) led to a series of SIRT1/2 inhibitors, in some cases more potent than 1a mainly against SIRT1. Tested in human leukemia U937 cells, the benzamide and anilide derivatives 1b, 1c, 2b, and 2c as well as the 4-(2-phenylpropyl)thioanalogue 4c showed huge apoptosis induction, while some sulfinyl and sulfonyl derivatives (5b, 5c, and 6a-c) were highly efficient in granulocytic differentiation. When assayed in human leukemia MOLT4 as well as in human breast MDA-MB-231 and colon RKO cancer cell lines, the anilide 2b (salermide) and the phenylpropylthio analogue 4b emerged as the most potent antiproliferative agents. Tested on colorectal carcinoma and glioblastoma multiforme cancer stem cells (CSCs) from patients, 2b was particularly potent against colorectal carcinoma CSCs, while 4b, 6a, and the SIRT2-selective inhibitor AGK-2 showed the highest effect against glioblastoma multiforme CSCs. Such compounds will be further explored for their broad-spectrum anticancer properties.

Benzodeazaoxaflavins as sirtuin inhibitors with antiproliferative properties in cancer stem cells

Inhibition of sirtuins has recently been proposed as a promising anticancer strategy. Some of the new benzodeazaoxaflavins (2a, 2b, and 2d) here reported as SIRT1/2 inhibitors were endowed with pro-apoptotic properties in human U937 leukemia cells and, most importantly, together with the prototype MC2141 (1) displayed antiproliferative effects in cancer stem cells from patients with colorectal carcinoma and glioblastoma multiforme, known to be highly tumorigenic, resistant to conventional cancer chemotherapy, and responsible, at least in part, for cancer relapse or recurrence.

Loss of histone deacetylase 4 causes segregation defects during mitosis of p53-deficient human tumor cells

We investigated the role of histone deacetylase 4 (HDAC4) using RNA interference (RNAi) and knockout cells to specifically address its role in cell cycle progression in tumor and normal cells. Ablation of HDAC4 led to growth inhibition in human tumor cells but not to detectable effects in normal human dermal fibroblasts (NHDF) or myelopoietic progenitors. HDAC4-/+ or HDAC4-/- murine embryonic fibroblasts showed no detectable growth defects. On the other hand, HDAC4 RNAi in HeLa cells produced mitotic arrest followed by caspase-dependent apoptosis. Mitotically arrested cells showed chromosome segregation defects. Even though the growth of both p53-wild-type and p53-null tumor cells were affected by HDAC4 ablation, segregation defects were observed only in p53-null cells. HDAC4 associates with the PP2A-B56 regulatory subunit, which is known to be involved in chromosome segregation, and RNAi of either the structural subunit A or the regulatory subunit B56 of PP2A also caused chromosome segregation defects. We conclude that HDAC4 is required for cell cycle progression of tumor cells by multiple mechanisms, one of which seems to be specific to p53-deficient cells through chromosome segregation defects. On the contrary, HDAC4 is not required for the progression of NHDF. We therefore suggest that systemic selective interference with the expression or function of HDAC4 is expected to have a significant therapeutic window, in particular, for p53-deficient tumors.

Inhibition of class I histone deacetylase with an apicidin derivative prevents cardiac hypertrophy and failure

AIMS

Recent studies have demonstrated the importance of chromatin remodelling via histone acetylation/deacetylation for the control of cardiac gene expression. Specific histone deacetylases (HDACs) can, in fact, play a positive or negative role in determining cardiac myocyte (CM) size. Here, we report on the effect on hypertrophy development of three inhibitors (HDACi) of class I HDACs.

METHODS AND RESULTS

The compounds were first analysed in vitro by scoring hypertrophy, expression of foetal genes, and apoptosis of neonatal rat CMs stimulated with phenylephrine, an alpha1-adrenergic agonist. This initial screening indicated that a truncated derivative of apicidin with class I HDAC specificity, denoted API-D, had the highest efficacy to toxicity ratio, and was thus selected for further analysis in vivo. Administration of this drug significantly decreased myocardial hypertrophy and foetal gene expression after 1 week of pressure overload induced by thoracic aortic constriction (TAC) in mice. After 9 weeks of TAC, when manifest heart failure is encountered, mice treated with API-D presented with significantly improved echocardiographic and haemodynamic parameters of cardiac function when compared with untreated TAC-operated mice.

CONCLUSION

The apicidin derivative, API-D, is capable of reducing hypertrophy and, consequently, the transition to heart failure in mice subjected to TAC. Treatment with this substance, therefore, holds promise as an important therapeutic option for heart failure.

Structural and functional analysis of the human HDAC4 catalytic domain reveals a regulatory structural zinc-binding domain

Histone deacetylases (HDACs) regulate chromatin status and gene expression, and their inhibition is of significant therapeutic interest. To date, no biological substrate for class IIa HDACs has been identified, and only low activity on acetylated lysines has been demonstrated. Here, we describe inhibitor-bound and inhibitor-free structures of the histone deacetylase-4 catalytic domain (HDAC4cd) and of an HDAC4cd active site mutant with enhanced enzymatic activity toward acetylated lysines. The structures presented, coupled with activity data, provide the molecular basis for the intrinsically low enzymatic activity of class IIa HDACs toward acetylated lysines and reveal active site features that may guide the design of class-specific inhibitors. In addition, these structures reveal a conformationally flexible structural zinc-binding domain conserved in all class IIa enzymes. Importantly, either the mutation of residues coordinating the structural zinc ion or the binding of a class IIa selective inhibitor prevented the association of HDAC4 with the N-CoR.HDAC3 repressor complex. Together, these data suggest a key role of the structural zinc-binding domain in the regulation of class IIa HDAC functions.

Probing the elusive catalytic activity of vertebrate class IIa histone deacetylases

It has been widely debated whether class IIa HDACs have catalytic deacetylase activity, and whether this plays any part in controlling gene expression. Herein, it has been demonstrated that class IIa HDACs isolated from mammalian cells are contaminated with other deacetylases, but can be prepared cleanly in Escherichia coli. These bacteria preparations have weak but measurable deacetylase activity. The low efficiency can be restored either by: mutation of an active site histidine to tyrosine, or by the use of a non-acetylated lysine substrate, allowing the development of assays to identify class IIa HDAC inhibitors.

PP2A regulates HDAC4 nuclear import

Different signal-regulated serine/threonine kinases phosphorylate class II histone deacetylases (HDACs) to promote nuclear export, cytosolic accumulation, and activation of gene transcription. However, little is known about mechanisms operating in the opposite direction, which, possibly through phosphatases, should promote class II HDACs nuclear entry and subsequent gene repression. Here we show that HDAC4 forms a complex with the PP2A holoenzyme C alpha, A alpha, B/PR55 alpha. In vitro and in vivo binding studies demonstrate that the N-terminus of HDAC4 interacts with the catalytic subunit of PP2A. HDAC4 is dephosphorylated by PP2A and experiments using okadaic acid or RNA interference have revealed that PP2A controls HDAC4 nuclear import. Moreover, we identified serine 298 as a putative phosphorylation site important for HDAC4 nuclear import. The HDAC4 mutant mimicking phosphorylation of serine 298 is defective in nuclear import. Mutation of serine 298 to alanine partially rescues the defect in HDAC4 nuclear import observed in cells with down-regulated PP2A. These observations suggest that PP2A, via the dephosphorylation of multiple serines including the 14-3-3 binding sites and serine 298, controls HDAC4 nuclear import.

Nitric Oxide Modulates Chromatin Folding in Human Endothelial Cells via Protein Phosphatase 2A Activation and Class II Histone Deacetylases Nuclear Shuttling

Nitric oxide (NO) modulates important endothelial cell (EC) functions and gene expression by a molecular mechanism which is still poorly characterized. Here we show that in human umbilical vein ECs (HUVECs) NO inhibited serum-induced histone acetylation and enhanced histone deacetylase (HDAC) activity. By immunofluorescence and Western blot analyses it was found that NO induced class II HDAC4 and 5 nuclear shuttling and that class II HDACs selective inhibitor MC1568 rescued serum-dependent histone acetylation above control level in NO-treated HUVECs. In contrast, class I HDACs inhibitor MS27–275 had no effect, indicating a specific role for class II HDACs in NO-dependent histone deacetylation. In addition, it was found that NO ability to induce HDAC4 and HDAC5 nuclear shuttling involved the activation of the protein phosphatase 2A (PP2A). In fact, HDAC4 nuclear translocation was impaired in ECs expressing small-t antigen and exposed to NO. Finally, in cells engineered to express a HDAC4-Flag fusion protein, NO induced the formation of a macromolecular complex including HDAC4, HDAC3, HDAC5, and an active PP2A. The present results show that NO-dependent PP2A activation plays a key role in class II HDACs nuclear translocation.

Substrate binding to histone deacetylases as shown by the crystal structure of the HDAC8-substrate complex

Histone deacetylases (HDACs)-an enzyme family that deacetylates histones and non-histone proteins-are implicated in human diseases such as cancer, and the first-generation of HDAC inhibitors are now in clinical trials. Here, we report the 2.0 A resolution crystal structure of a catalytically inactive HDAC8 active-site mutant, Tyr306Phe, bound to an acetylated peptidic substrate. The structure clarifies the role of active-site residues in the deacetylation reaction and substrate recognition. Notably, the structure shows the unexpected role of a conserved residue at the active-site rim, Asp 101, in positioning the substrate by directly interacting with the peptidic backbone and imposing a constrained cis-conformation. A similar interaction is observed in a new hydroxamate inhibitor-HDAC8 structure that we also solved. The crucial role of Asp 101 in substrate and inhibitor recognition was confirmed by activity and binding assays of wild-type HDAC8 and Asp101Ala, Tyr306Phe and Asp101Ala/Tyr306Phe mutants.

HDACs, histone deacetylation and gene transcription: from molecular biology to cancer therapeutics

Histone deacetylases (HDACs) and histone acetyl transferases (HATs) are two counteracting enzyme families whose enzymatic activity controls the acetylation state of protein lysine residues, notably those contained in the N-terminal extensions of the core histones. Acetylation of histones affects gene expression through its influence on chromatin conformation. In addition, several non-histone proteins are regulated in their stability or biological function by the acetylation state of specific lysine residues. HDACs intervene in a multitude of biological processes and are part of a multiprotein family in which each member has its specialized functions. In addition, HDAC activity is tightly controlled through targeted recruitment, protein-protein interactions and post-translational modifications. Control of cell cycle progression, cell survival and differentiation are among the most important roles of these enzymes. Since these processes are affected by malignant transformation, HDAC inhibitors were developed as antineoplastic drugs and are showing encouraging efficacy in cancer patients.

Molecular insights into azumamide e histone deacetylases inhibitory activity

Azumamide E, a cyclotetrapeptide isolated from the sponge Mycale izuensis, is the most powerful carboxylic acid containing natural histone deacetylase (HDAC) inhibitor known to date. In this paper, we describe design and synthesis of two stereochemical variants of the natural product. These compounds have allowed us to clarify the influence of side chain topology on the HDAC-inhibitory activity. The present contribution also reveals the identity of the recognition pattern between azumamides and the histone deacetylase-like protein (HDLP) model receptor and reports the azumamide E unprecedented isoform selectivity on histone deacetylases class subtypes. From the present studies, a plausible model for the interaction of azumamides with the receptor binding pocket is derived, providing a framework for the rational design of new cyclotetrapeptide-based HDAC inhibitors as antitumor agents.

Precision recall with user modeling (PRUM)

Standard Information Retrieval (IR) metrics are not well suited for new paradigms like XML or Web IR in which retrievable information units are document elements and/or sets of related documents. Part of the problem stems from the classical hypotheses on the user models: They do not take into account the structural or logical context of document elements or the possibility of navigation between units. This article proposes an explicit and formal user model that encompasses a large variety of user behaviors. Based on this model, we extend the probabilistic precision-recall metric to deal with the new IR paradigms.

Functional and morphological recovery of dystrophic muscles in mice treated with deacetylase inhibitors

Pharmacological interventions that increase myofiber size counter the functional decline of dystrophic muscles. We show that deacetylase inhibitors increase the size of myofibers in dystrophin-deficient (MDX) and alpha-sarcoglycan (alpha-SG)-deficient mice by inducing the expression of the myostatin antagonist follistatin in satellite cells. Deacetylase inhibitor treatment conferred on dystrophic muscles resistance to contraction-coupled degeneration and alleviated both morphological and functional consequences of the primary genetic defect. These results provide a rationale for using deacetylase inhibitors in the pharmacological therapy of muscular dystrophies.

A Structure-Guided Approach to an Orthogonal Estrogen-Receptor-Based Gene Switch Activated by Ligands Suitable for in Vivo Studies

A strategy to obtain a fully orthogonal estrogen-receptor-based gene switch responsive to molecules with acceptable pharmacological properties is presented. From a series of tetrahydrofluorenones active on the wild-type estrogen receptor (ER) an inactive analogue is chosen as a new lead compound. Coevolution of receptor mutants and ligands leads to an ER-based gene switch suitable for studies in animal models.

A functionally orthogonal estrogen receptor-based transcription switch specifically induced by a nonsteroid synthetic ligand

It is highly desirable to design ligand-dependent transcription regulation systems based on transactivators unresponsive to endogenous ligands but induced by synthetic small molecules unable to activate endogenous receptors. Using molecular modeling and yeast selection, we identified an estrogen receptor ligand binding domain double mutant (L384M, M421G) with decreased affinity to estradiol and enhanced binding to compounds inactive on estrogen receptors. Nonresponsiveness to estrogen was achieved by additionally adding the G521R substitution while introducing an "antagonistic-type" side chain in the compound, as in 4-hydroxytamoxifen. The triple-substituted ligand binding domain is insensitive to physiological concentrations of estradiol and has nanomolar affinity for the ligand. In this binary system, both receptor and ligand are, therefore, reciprocally specific. The mutated variant in the context of a chimeric transcription factor provides tight, ligand-dependent regulation of reporter gene expression.

Characterization of prejunctional serotonin receptors modulating [3H]acetylcholine release in the human detrusor

Bladder overactivity (OAB) is a chronic and debilitating lower urinary tract (LUT) disorder that affects millions of individuals worldwide. LUT symptoms associated with OAB, such as urgency and urinary incontinence, cause a hygienic and social concern to patients, but their current pharmacological treatment is largely inadequate due to the lack of uroselectivity. Although OAB etiology remains multifactorial and poorly understood, increasing evidence indicates that serotonin [5-hydroxytryptamine (5-HT)] is an endogenous substance involved in the control of micturition at central and peripheral sites. In this study, we demonstrated the presence of three distinct 5-HT receptors localized at parasympathetic nerve terminals of the human bladder by measuring electrically evoked tritiated acetylcholine release in isolated detrusor strips. These prejunctional receptors, involved in both positive and negative feedback mechanisms regulating cholinergic transmission, have been characterized by means of three highly selective 5-HT antagonists for 5-HT(4), 5-HT(7), and 5-HT(1A) receptors, namely GR113808A ([1-[2-[(-methylsulphonyl) amino] ethyl]4-piperinidyl]methyl1-methyl-1H-indole-3-carboxylate succinate), SB269970 [(R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl)phenol hydrochloride], and WAY100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)-cyclohexane-carboxamide trichloride]. Under these conditions, we confirmed the facilitatory role of 5-HT(4) heteroreceptors on acetylcholine release and revealed for the first time the occurrence of 5-HT(7) and 5-HT(1A) heteroreceptors with a facilitatory and an inhibitory action, respectively. Our findings strengthen the novel concept for the use of recently patented selective 5-HT agonists and antagonists for the control of OAB dysfunctions associated with inflammatory conditions, although their therapeutic efficacy needs to be explored in the clinical setting.

Crystal structure of a eukaryotic zinc-dependent histone deacetylase, human HDAC8, complexed with a hydroxamic acid inhibitor

Histone deacetylases (HDACs) are a family of enzymes involved in the regulation of gene expression, DNA repair, and stress response. These processes often are altered in tumors, and HDAC inhibitors have had pronounced antitumor activity with promising results in clinical trials. Here, we report the crystal structure of human HDAC8 in complex with a hydroxamic acid inhibitor. Such a structure of a eukaryotic zinc-dependent HDAC has not be described previously. Similar to bacterial HDAC-like protein, HDAC8 folds in a single alpha/beta domain. The inhibitor and the zinc-binding sites are similar in both proteins. However, significant differences are observed in the length and structure of the loops surrounding the active site, including the presence of two potassium ions in HDAC8 structure, one of which interacts with key catalytic residues. CD data suggest a direct role of potassium in the fold stabilization of HDAC8. Knockdown of HDAC8 by RNA interference inhibits growth of human lung, colon, and cervical cancer cell lines, highlighting the importance of this HDAC subtype for tumor cell proliferation. Our findings open the way for the design and development of selective inhibitors of HDAC8 as possible antitumor agents.

Biochemical and immunologic properties of the nonstructural proteins of the hepatitis C virus: implications for development of antiviral agents and vaccines

Infection with the hepatitis C virus (HCV) is the major cause of non-A, non-B hepatitis worldwide. The viral genome, a positive-sense, single-stranded, 9.6-kb long RNA molecule, is translated into a single polyprotein of about 3,000 amino acids. The viral polyprotein is proteoytically processed to yield all the mature viral gene products. The genomic order of HCV has been determined to be C-->E1-->E2-->p7-->NS2-->NS3-->NS4A-->NS4B-->NS5A++ +-->NS5B. C, E1, and E2 are the virion structural proteins. Whereas the function of p7 is currently unknown, NS2 to NS5B are thought to be the nonstructural proteins. Generation of the mature nonstructural proteins relies on the activity of viral proteinases. Cleavage at the NS2-NS3 junction is accomplished by a metal-dependent autocatalytic proteinase encoded within NS2 and the N-terminus of NS3. The remaining downstream cleavages are effected by a serine proteinase contained also within the N-terminal region of NS3. NS3, in addition, contains an RNA helicase domain at its C-terminus. NS3 forms a heterodimeric complex with NS4A. The latter is a membrane protein that acts as a cofactor of the proteinase. Although no function has yet been attributed to NS4B, NS5A has been recently suggested to be involved in mediating the resistance of the HCV to the action of interferon. Finally, the NS5B protein has been shown to be the viral RNA-dependent RNA polymerase. This article reviews the current understanding of the structure and the function of the various HCV nonstructural proteins with particular emphasis on their potential as targets for the development of novel antiviral agents and vaccines.

Mutational analysis of hepatitis C virus NS3-associated helicase

Nonstructural protein 3 (NS3) of hepatitis C virus contains a bipartite structure consisting of an N-terminal serine protease and a C-terminal DEXH box helicase. To investigate the roles of individual amino acid residues in the overall mechanism of unwinding, a mutational-functional analysis was performed based on a molecular model of the NS3 helicase domain bound to ssDNA, which has largely been confirmed by a recently published crystal structure of the NS3 helicase-ssDNA complex. Three full-length mutated NS3 proteins containing Tyr(392)Ala, Val(432)Gly and Trp(501)Ala single substitutions, respectively, together with a Tyr(392)Ala/Trp(501)Ala double-substituted protein were expressed in Escherichia coli and purified to homogeneity. All individually mutated forms showed a reduction in duplex unwinding activity, single-stranded polynucleotide binding capacity and polynucleotide-stimulated ATPase activity compared to wild-type, though to different extents. Simultaneous replacement of both Tyr(392) and Trp(501) with Ala completely abolished all these enzymatic functions. On the other hand, the introduced amino acid substitutions had no influence on NS3 intrinsic ATPase activity and proteolytic efficiency. The results obtained with Trp(501)Ala and Val(432)Gly single-substituted enzymes are in agreement with a recently proposed model for NS3 unwinding activity. The mutant phenotype of the Tyr(392)Ala and Tyr(392)Ala/Trp(501)Ala enzymes, however, represents a completely novel finding.

Enzymatic properties of hepatitis C virus NS3-associated helicase

The hepatitis C virus non-structural protein 3 (NS3) possesses a serine protease activity in the N-terminal one-third, whereas RNA-stimulated NTPase and helicase activities reside in the C-terminal portion. In this study, an N-terminal hexahistidine-tagged full-length NS3 polypeptide was expressed in Escherichia coli and purified to homogeneity by conventional chromatography. Detailed characterization of the helicase activity of NS3 is presented with regard to its binding and strand release activities on different RNA substrates. On RNA double-hybrid substrates, the enzyme was shown to perform unwinding activity starting from an internal ssRNA region of at least 3 nt and moving along the duplex in a 3' to 5' direction. In addition, data are presented suggesting that binding to ATP reduces the affinity of NS3 for ssRNA and increases its affinity for duplex RNA. Furthermore, we have ascertained the capacity of NS3 to specifically interact with and resolve the stem-loop RNA structure (SL I) within the 3'-terminal 46 bases of the viral genome. Finally, our analysis of NS3 processive unwinding under single cycle conditions by addition of heparin in both helicase and RNA-stimulated ATPase assays led to two conclusions: (i) NS3-associated helicase acts processively; (ii) most of the NS3 RNA-stimulated ATPase activity may not be directly coupled to translocation of the enzyme along the substrate RNA molecule.

Improved performance in protein secondary structure prediction by inhomogeneous score combination

MOTIVATION

In many fields of pattern recognition, combination has proved efficient to increase the generalization performance of individual prediction methods. Numerous systems have been developed for protein secondary structure prediction, based on different principles. Finding better ensemble methods for this task may thus become crucial. Furthermore, efforts need to be made to help the biologist in the post-processing of the outputs.

RESULTS

An ensemble method has been designed to post-process the outputs of discriminant models, in order to obtain an improvement in prediction accuracy while generating class posterior probability estimates. Experimental results establish that it can increase the recognition rate of protein secondary structure prediction methods that provide inhomogeneous scores, even though their individual prediction successes are largely different. This combination thus constitutes a help for the biologist, who can use it confidently on top of any set of prediction methods. Moreover, the resulting estimates can be used in various ways, for instance to determine which areas in the sequence are predicted with a given level of reliability.

AVAILABILITY

The prediction is freely available over the Internet on the Network Protein Sequence Analysis (NPS@) WWW server at http://pbil.ibcp.fr/NPSA/npsa_server.ht ml. The source code of the combiner can be obtained on request for academic use.

Modulation of hepatitis C virus NS3 protease and helicase activities through the interaction with NS4A

The hepatitis C virus nonstructural 3 protein (NS3) possesses a serine protease activity in the N-terminal one-third, whereas RNA-stimulated NTPase and helicase activities reside in the C-terminal portion. The serine protease activity is required for proteolytic processing at the NS3-NS4A, NS4A-NS4B, NS4B-NS5A, and NS5A-NS5B polyprotein cleavage sites. NS3 forms a complex with NS4A, a 54-residue polypeptide that was shown to act as an essential cofactor of the NS3 protease. We have expressed in Escherichia coli the NS3-NS4A precursor; cleavage at the junction between NS3 and NS4A occurs during expression in the bacteria cells, resulting in the formation of a soluble noncovalent complex with a sub-nanomolar dissociation constant. We have assessed the minimal ionic strength and detergent and glycerol concentrations required for maximal proteolytic activity and stability of the purified NS3-NS4A complex. Using a peptide substrate derived from the NS5A-NS5B junction, the catalytic efficiency (kcat/Km) of NS3-NS4A-associated protease under optimized conditions was 55 000 s-1 M-1, very similar to that measured with a recombinant complex purified from eukaryotic cells. Dissociation of the NS3-NS4A complex was found to be fully reversible. No helicase activity was exhibited by the purified NS3-NS4A complex, but NS3 was fully active as a helicase upon dissociation of NS4A. On the other hand, both basal and poly(U)-induced NTPase activity and ssRNA binding activity associated with the NS3-NS4A complex were very similar to those exhibited by NS3 alone. Therefore, NS4A appears to uncouple the ATPase/ssRNA binding and RNA unwinding activities associated with NS3.

Multiple determinants influence complex formation of the hepatitis C virus NS3 protease domain with its NS4A cofactor peptide

The interaction of the hepatitis C virus (HCV) NS3 protease domain with its NS4A cofactor peptide (Pep4AK) was investigated at equilibrium and at pre-steady state under different physicochemical conditions. Equilibrium dissociation constants of the NS3-Pep4AK complex varied by several orders of magnitude depending on buffer additives. Glycerol, NaCl, detergents, and peptide substrates were found to stabilize this interaction. The extent of glycerol-induced stabilization varied in an HCV strain-dependent way with at least one determinant mapping to an NS3-NS4A interaction site. Conformational transitions affecting at least the first 18 amino acids of NS3 were the main energy barriers for both the association and the dissociation reactions of the complex. However, deletion of this N-terminal portion of the protease molecule only slightly influenced equilibrium dissociation constants determined under different physicochemical conditions. Limited proteolysis experiments coupled with mass spectrometric identification of cleavage fragments suggested a high degree of conformational flexibility affecting at least the first 21 residues of NS3. The accessibility of this region of the protease to limited chymotryptic digestion did not significantly change in any condition tested, whereas a significant reduction of chymotryptic cleavages within the NS3 core was detected under conditions of high NS3-Pep4AK complex affinity. We conclude the following: (1) The N-terminus of the NS3 protease that, according to the X-ray crystal structure, makes extensive contacts with the cofactor peptide is highly flexible in solution and contributes only marginally to the thermodynamic stability of the complex. (2) Affinity enhancement is accomplished by several factors through a general stabilization of the fold of the NS3 molecule.

Human thymine DNA glycosylase binds to apurinic sites in DNA but is displaced by human apurinic endonuclease 1

In vitro, following the removal of thymine from a G.T mismatch, thymine DNA glycosylase binds tightly to the apurinic site it has formed. It can also bind to an apurinic site opposite S6-methylthioguanine (SMeG) or opposite any of the remaining natural DNA bases. It will therefore bind to apurinic sites formed by spontaneous depurination, chemical attack, or other glycosylases. In the absence of magnesium, the rate of dissociation of the glycosylase from such complexes is so slow (koff 1.8 - 3.6 x 10(-5) s-1; i.e. half-life between 5 and 10 h) that each molecule of glycosylase removes essentially only one molecule of thymine. In the presence of magnesium, the dissociation rates of the complexes with C.AP and SMeG.AP are increased more than 20-fold, allowing each thymine DNA glycosylase to remove more than one uracil or thymine from C.U and SMeG.T mismatches in DNA. In contrast, magnesium does not increase the dissociation of thymine DNA glycosylase from G.AP sites sufficiently to allow it to remove more than one thymine from G.T mismatches. The bound thymine DNA glycosylase prevents human apurinic endonuclease 1 (HAP1) cutting the apurinic site, so unless the glycosylase was displaced, the repair of apurinic sites would be very slow. However, HAP1 significantly increases the rate of dissociation of thymine DNA glycosylase from apurinic sites, presumably through direct interaction with the bound glycosylase. This effect is concentration-dependent and at the probable normal concentration of HAP1 in cells the dissociation would be fast. This interaction couples the first step in base excision repair, the glycosylase, to the second step, the apurinic endonuclease. The other proteins involved in base excision repair, polymerase beta, XRCC1, and DNA ligase III, do not affect the dissociation of thymine DNA glycosylase from the apurinic site.

Multiple enzymatic activities associated with recombinant NS3 protein of hepatitis C virus

The hepatitis C virus (HCV) nonstructural 3 protein (NS3) contains at least two domains associated with multiple enzymatic activities; a serine protease activity resides in the N-terminal one-third of the protein, whereas RNA helicase activity and RNA-stimulated nucleoside triphosphatase activity are associated with the C-terminal portion. To study the possible mutual influence of these enzymatic activities, a full-length NS3 polypeptide of 67 kDa was expressed as a nonfusion protein in Escherichia coli, purified to homogeneity, and shown to retain all three enzymatic activities. The protease activity of the full-length NS3 was strongly dependent on the activation by a synthetic peptide spanning the central hydrophobic core of the NS4A cofactor. Once complexed with the NS4A-derived peptide, the full-length NS3 protein and the isolated N-terminal protease domain cleaved synthetic peptide substrates with comparable efficiency. We show that, as in the case of the isolated protease domain, the protease activity of full-length NS3 undergoes inhibition by the N-terminal cleavage products of substrate peptides corresponding to the NS4A-NS4B and NS5A-NS5B. We have also characterized and quantified the NS3 ATPase, RNA helicase, and RNA-binding activities under optimized reaction conditions. Compared with the isolated N-terminal and C-terminal domains, recombinant full-length NS3 did not show significant differences in the three enzymatic activities analyzed in independent in vitro assays. We have further explored the possible interdependence of the NS3 N-terminal and C-terminal domains by analyzing the effect of polynucleotides on the modulation of all NS3 enzymatic functions. Our results demonstrated that the observed inhibition of the NS3 proteolytic activity by single-stranded RNA is mediated by direct interaction with the protease domain rather than with the helicase RNA-binding domain.

Expression of long-patch and short-patch DNA mismatch repair proteins in the embryonic and adult mammalian brain

Expression of the DNA mismatch repair (MMR) pathway was examined in the adult and developing rat brain. Rat homologues of human GTBP and MSH2, which are essential components of the post-replicative DNA MMR system, were identified in nuclear extracts from the adult and developing rat brain. Developmental studies showed that both GTBP and MSH2 levels were higher in nuclei isolated from the embryonic brain (day 16) than adult brain. However, this difference was not as dramatic as the difference in the number of proliferating cells. Levels of thymine DNA glycosylase (TDG), the enzyme which catalyzes the first step in short patch G:T mismatch repair, were also decreased in adult compared to embryonic brain. In the adult brain, MMR proteins were elevated in nuclear extracts enriched for neuronal nuclei. These results suggest that adult brain cells have the capacity to carry out DNA mismatch repair, in spite of a lack of ongoing DNA replication.

Chromosomal localizations and molecular analysis of TDG gene-related sequences

The human mismatch-specific thymine DNA glycosylase gene, TDG, encodes a 60-kDa polypeptide able to correct G/T mispairs arising from the deamination of 5-methylcytosine. We localized by FISH three different TDG-related lambda genomic clones, lambda8, lambda11, and lambda12 on chromosome 12. PCR and sequence analyses revealed that only lambda11, localized at 12q24.1, contained the coding gene. We characterized the intron-exon boundaries of the portion of the gene contained in the lambda clone and identified a CA dinucleotide repeat in one intron. Northern blot analysis showed that TDG is expressed at approximately the same level in all human tissues analyzed. SSCP analysis of 50 tumor and corresponding normal tissue DNAs from lung cancer patients did not reveal the presence of any functional mutation. An abnormal SSCP pattern in one sample proved to be a polymorphism after sequencing and RFLP analysis.

The nonstructural proteins of the hepatitis C virus: structure and functions

The hepatitis C virus is the major causative agent of nonA-nonB hepatitis worldwide. Although this virus cannot be cultivated in cell culture, several of its features have been elucidated in the past few years. The viral genome is a single-stranded, 9.5kb long RNA molecule of positive polarity. The viral genome is translated into a single polyprotein of about 3000 amino acids. The virally encoded polyprotein undergoes proteolytic processing by a combination of cellular and viral proteolytic enzymes in order to yield all the mature viral gene products. The gene order of HCV has been determined to be C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B. The mature structural proteins, C, E1 and E2 have been shown to arise from the viral polyprotein via proteolytic processing by host signal peptidases. Conversely, generation of the mature nonstructural proteins relies on the activity of viral proteases. Thus, cleavage at the NS2/NS3 junction is accomplished by a metal-dependent autoprotease encoded within NS2 and the N-terminus of NS3. The remaining cleavages downstream from this site are effected by a serine protease contained within the N-terminal region of NS3. Besides the protease domain, NS3 also contains an RNA helicase domain at its C-terminus. NS3 forms a heterodimeric complex with NS4A. The latter is a membrane protein that has been shown to act as a cofactor of the protease. Whereas the NS5B protein has been shown to be the viral RNA-dependent RNA polymerase, no function has yet been attributed to NS4B and NS5A. The latter is a cytoplasmic phosphoprotein and appears to be involved in mediating the resistance of the hepatitis C virus to the action of interferon.

Investigation of the mechanisms of DNA binding of the human G/T glycosylase using designed inhibitors

Deamination of 5-methylcytosine residues in DNA gives rise to the G/T mismatched base pair. In humans this lesion is repaired by a mismatch-specific thymine DNA glycosylase (TDG or G/T glycosylase), which catalyzes specific excision of the thymine base through N-glycosidic bond hydrolysis. Unlike other DNA glycosylases, TDG recognizes an aberrant pairing of two normal bases rather than a damaged base per se. An important structural issue is thus to understand how the enzyme specifically targets the T (or U) residue of the mismatched base pair. Our approach toward the study of substrate recognition and processing by catalytic DNA binding proteins has been to modify the substrate so as to preserve recognition of the base but to prevent its excision. Here we report that replacement of 2'-hydrogen atoms with fluorine in the substrate 2'-deoxyguridine (dU) residue abrogates glycosidic bond cleavage, thereby leading to the formation of a tight, specific glycosylase-DNA complex. Biochemical characterization of these complexes reveals that the enzyme protects an approximately 20-bp stretch of the substrate from DNase I cleavage, and directly contacts a G residue on the 3' side of the mismatched U derivative. These studies provide a mechanistic rationale for the preferential repair of deaminated CpG sites and pave the way for future high-resolution studies of TDG bound to DNA.

A new class of uracil-DNA glycosylases related to human thymine-DNA glycosylase

Mispairs in DNA of guanine with uracil and thymine can arise as a result of deamination of cytosine and 5-methylcytosine, respectively. In humans such mispairs are removed by thymine-DNA glycosylase (TDG). By deleting the carboxy and amino termini of this enzyme we have identified a core region capable of processing G/U but not G/T mispairs. We have further identified two bacterial proteins with strong sequence homology to this core and shown that the homologue from Escherichia coli (dsUDG) can remove uracil from G/U mispairs. This enzyme is likely to act as a back-up to the highly efficient and abundant enzyme uracil-DNA glycosylase (UDG) which is found in most organisms. Pupating insects have been reported to lack UDG activity, but we have identified an enzyme similar to dsUDG in cell lines from three different insect species. These data imply the existence of a family of double-strand-specific uracil-DNA glycosylases which, although they are subservient to UDG in most organisms, may constitute the first line of defence against the mutagenic effects of cytosine deamination in insects.

Base analog and neighboring base effects on substrate specificity of recombinant human G:T mismatch-specific thymine DNA-glycosylase

We studied the substrate specificity of the human G:T mismatch-specific thymine glycosylase that initiates the repair of G:T and G:U base mismatches to G:C base pairs. Such mismatches arise when 5-methylcytosine or cytosine deaminate spontaneously (and hydrolytically) in DNA. Substrates were 45-bp DNA heteroduplexes that bore single G:T, m6G:T, 2,6-diaminopurine:T, 2-amino-6-(methylamino)-purine:T, 2-aminopurine:T, and G:m4T mispairs. The bases 5' to the poorly matched G were altered in selected G:T substrates to yield mispairs in four different contexts, ApG, CpG, GpG, and TpG. The recombinant thymine glycosylase was incubated with the 45-bp DNA substrates, each labeled at the 5'-terminus of the strand containing the mismatched T. The DNAs were then treated with 0.1 N NaOH to catalyze phosphodiester bond breakage at the newly-generated AP sites, and the products were analyzed on DNA sequencing gels. As indicated by the amounts of the 20-nt incision product, the removal of the thymine base by the enzyme increased linearly between 0 and 40 min at which time the generation of product from all substrates ceased, probably because of enzyme inactivation. The rate of incision was greatest (0.7 fmol/min) with DNA containing the G:T mispair followed by the DNA containing the m6G:T mispair (0.38 fmol/min) and the DNA with the 2-amino-6-(methylamino)purine:T mispair (0.15 fmol/ min); the extent of reaction was 90%, 40%, and 20% respectively. By contrast to previous findings with cell-free extracts, DNA substrates containing 2,6-diaminopurine:T, 2-aminopurine:T, and G:m4T mispairs were not incised (< 2%). The amount of incision of the 45-bp DNA substrates containing G:T mispairs in the CpG context was 3-12-fold greater than in the TpG, GpG, and ApG contexts.

Cloning and expression of human G/T mismatch-specific thymine-DNA glycosylase

Hydrolytic deamination of 5-methylcytosine leads to the formation of G/T mismatches. We have shown previously that these G/T mispairs are corrected to G/C pairs by a mismatch-specific thymine-DNA glycosylase, TDG, which we subsequently purified from human cells. Here we describe the cloning of the human cDNA encoding TDG. We have identified two distinct cDNA species that differ by 100 nucleotides at the 3'-untranslated region. These cDNAs contain a 410-amino acid open reading frame that encodes a 46-kDa polypeptide. The G/T glycosylase, expressed both in vitro and in Escherichia coli, migrated in denaturing polyacrylamide gels with an apparent size of 60 kDa. The substrate specificity of the recombinant protein corresponded to that of the cellular enzyme, and polyclonal antisera raised against the recombinant protein neutralized both activities. We therefore conclude that the cDNA described below encodes human TDG. Data base searches identified a serendipitously cloned mouse cDNA sequence that could be shown to encode the murine TDG homologue. No common amino acid sequence motifs between the G/T-specific enzyme and other DNA glycosylases could be found, suggesting that TDG belongs to a new class of base-excision repair enzymes.

GTBP, a 160-kilodalton protein essential for mismatch-binding activity in human cells

DNA mismatch recognition and binding in human cells has been thought to be mediated by the hMSH2 protein. Here it is shown that the mismatch-binding factor consists of two distinct proteins, the 100-kilodalton hMSH2 and a 160-kilodalton polypeptide, GTBP (for G/T binding protein). Sequence analysis identified GTBP as a new member of the MutS homolog family. Both proteins are required for mismatch-specific binding, a result consistent with the finding that tumor-derived cell lines devoid of either protein are also devoid of mismatch-binding activity.

Localization of a 34-amino-acid segment implicated in dimerization of the herpes simplex virus type 1 ICP4 polypeptide by a dimerization trap

The herpes simplex virus type 1 immediate-early protein ICP4 plays an essential role in the regulation of the expression of all viral genes. It is the major trans activator of early and late genes and also has a negative regulatory effect on immediate-early gene transcription. ICP4 is a sequence-specific DNA-binding protein and has always been purified in a dimeric form. The part of the protein that consists of the entire highly conserved region 2 and of the distal portion of region 1 retains the ability to specifically associate with DNA and to form homodimers in solution. In an attempt to map the dimerization domain of ICP4, we used a dimerization trap assay, in which we screened deletion fragments of this 217-amino-acid stretch for sequences that could confer dimerization properties on a heterologous cellular transcription factor (LFB1), which binds to its cognate DNA sequence only as a dimer. The analysis of these chimeric proteins expressed in vitro ultimately identified a stretch of 34 amino acids (343 to 376) that could still confer DNA-binding activity on the LFB1 reporter protein and thus apparently contained the ICP4 dimerization motif. Consistent with this result, a truncated ICP4 protein containing amino acids 343 to 490, in spite of the complete loss of DNA-binding activity, appeared to retain the capacity to form a heterodimer with a longer ICP4 peptide after coexpression in an in vitro translation system.