Dysregulation of hsa-miR-34a and hsa-miR-449a leads to overexpression of PACS-1 and loss of DNA damage response (DDR) in cervical cancer.

We have observed overexpression of PACS-1, a cytosolic sorting protein in primary cervical tumors. Absence of exonic mutations and overexpression at the RNA level suggested a transcriptional and/or posttranscriptional regulation. University of California Santa Cruz genome browser analysis of PACS-1 micro RNAs (miR), revealed two 8-base target sequences at the 3' terminus for hsa-miR-34a and hsa-miR-449a. Quantitative RT-PCR and Northern blotting studies showed reduced or loss of expression of the two microRNAs in cervical cancer cell lines and primary tumors, indicating dysregulation of these two microRNAs in cervical cancer. Loss of PACS-1 with siRNA or exogenous expression of hsa-miR-34a or hsa-miR-449a in HeLa and SiHa cervical cancer cell lines resulted in DNA damage response, S-phase cell cycle arrest, and reduction in cell growth. Furthermore, the siRNA studies showed that loss of PACS-1 expression was accompanied by increased nuclear γH2AX expression, Lys382-p53 acetylation, and genomic instability. PACS-1 re-expression through LNA-hsa-anti-miR-34a or -449a or through PACS-1 cDNA transfection led to the reversal of DNA damage response and restoration of cell growth. Release of cells post 24-h serum starvation showed PACS-1 nuclear localization at G1-S phase of the cell cycle. Our results therefore indicate that the loss of hsa-miR-34a and hsa-miR-449a expression in cervical cancer leads to overexpression of PACS-1 and suppression of DNA damage response, resulting in the development of chemo-resistant tumors.

A cell-permeable hairpin peptide inhibits hepatitis C viral nonstructural protein 5A-mediated translation and virus production.

UNLABELLED: NS5A is a key regulator of the hepatitis C virus (HCV) life cycle including RNA replication, assembly, and translation. We and others have shown that NS5A augments HCV internal ribosomal entry site (IRES)-mediated translation. Furthermore, Quercetin treatment and heat shock protein (HSP) 70 knockdown inhibit the NS5A-driven augmentation of IRES-mediated translation and infectious virus production. We have also coimmunoprecipitated HSP70 with NS5A and demonstrated cellular colocalization, leading to the hypothesis that the NS5A/HSP70 complex formation is important for IRES-mediated translation. Here, we have identified the NS5A region responsible for complex formation through in vitro deletion analyses. Deletion of NS5A domains II and III failed to reduce HSP70 binding, whereas domain I deletion eliminated complex formation. NS5A domain I alone also bound HSP70. Deletion mapping of domain I identified the C-terminal 34 amino acids (C34) as the interaction site. Furthermore, addition of C34 to domains II and III restored complex formation. C34 expression significantly reduced intracellular viral protein levels, in contrast to same-size control peptides from other NS5A domains. C34 also competitively inhibited NS5A-augmented IRES-mediated translation, whereas controls did not. Triple-alanine scan mutagenesis determined that an exposed beta-sheet hairpin in C34 was primarily responsible for NS5A-augmented IRES-mediated translation. Moreover, treatment with a 10-amino acid peptide derivative of C34 suppressed NS5A-augmented IRES-mediated translation and significantly inhibited intracellular viral protein synthesis, with no associated cytotoxicity. CONCLUSION: These results support the hypothesis that the NS5A/HSP70 complex augments viral IRES-mediated translation, identify a sequence-specific hairpin element in NS5A responsible for complex formation, and demonstrate the functional significance of C34 hairpin-mediated NS5A/HSP70 interaction. Identification of this element may allow for further interrogation of NS5A-mediated IRES activity, sequence-specific HSP recognition, and rational drug design. (HEPATOLOGY 2012;55:1662-1672).

The heat shock protein inhibitor Quercetin attenuates hepatitis C virus production.

UNLABELLED: The hepatitis C viral (HCV) genome is translated through an internal ribosome entry site (IRES) as a single polyprotein precursor that is subsequently cleaved into individual mature viral proteins. Nonstructural protein 5A (NS5A) is one of these proteins that has been implicated in regulation of viral genome replication, translation from the viral IRES and viral packaging. We sought to identify cellular proteins that interact with NS5A and determine whether these interactions may play a role in viral production. Mass spectrometric analysis of coimmunoprecipitated NS5A complexes from cell extracts identified heat shock proteins (HSPs) 40 and 70. We confirmed an NS5A/HSP interaction by confocal microscopy demonstrating colocalization of NS5A with HSP40 and with HSP70. Western analysis of coimmunoprecipitated NS5A complexes further confirmed interaction of HSP40 and HSP70 with NS5A. A transient transfection, luciferase-based, tissue culture IRES assay demonstrated NS5A augmentation of HCV IRES-mediated translation, and small interfering RNA (siRNA)-mediated knockdown of HSP70 reduced this augmentation. Treatment with an inhibitor of HSP synthesis, Quercetin, markedly reduced baseline IRES activity and its augmentation by NS5A. HSP70 knockdown also modestly reduced viral protein accumulation, whereas HSP40 and HSP70 knockdown both reduced infectious viral particle production in an HCV cell culture system using the J6/JFH virus fused to the Renilla luciferase reporter. Treatment with Quercetin reduced infectious particle production at nontoxic concentrations. The marked inhibition of virus production by Quercetin may partially be related to reduction of HSP40 and HSP70 and their potential involvement in IRES translation, as well as viral morphogenesis or secretion. CONCLUSION: Quercetin may allow for dissection of the viral life cycle and has potential therapeutic use to reduce virus production with low associated toxicity.

The interaction of cytoplasmic RNA viruses with the nucleus.

Mammalian cells infected with poliovirus, the prototype member of the picornaviridae family, undergo rapid macromolecular and metabolic changes resulting in efficient replication and release of virus from infected cells. Although this virus is predominantly cytoplasmic, it does shut-off transcription of all three cellular transcription systems. Both biochemical and genetic studies have shown that a virally encoded protease, 3C(pro), is responsible for host cell transcription shut-off. The 3C protease cleaves a number of RNA polymerase II transcription factors including the TATA-binding protein (TBP), the cyclic AMP-responsive element binding protein (CREB), the Octamer binding protein (Oct-1), p53, and RNA polymerase III transcription factor IIICalpha, and Polymerase I factor SL-1. Most of these cleavages occur at glutamine-glycine bonds. Additionally, a second viral protease, 2A(pro), also cleaves TBP at a tyrosine-glycine bond. The latter cleavage could be responsible for shut-off of small nuclear RNA transcription. Recent studies indicate that the viral protease-polymerase precursor 3CD can enter nucleus in poliovirus-infected cells. The nuclear localization signal (NLS) present within the 3D sequence appears to play a role in the nuclear entry of 3CD. Thus, 3C may be delivered to the infected cell nucleus in the form the precursor 3CD or other 3C-containing precursors. Auto-proteolytic cleavage of these precursors could then generate 3C. Thus, for a small RNA virus that strictly replicates in the cytoplasm, a portion of its life cycle does include interaction with the host cell nucleus.

The NS5A-binding heat shock proteins HSC70 and HSP70 play distinct roles in the hepatitis C viral life cycle.

We previously identified HSP70 and HSC70 in complex with NS5A in a proteomic screen. Here, coimmunoprecipitation studies confirmed NS5A/HSC70 complex formation during infection, and immunofluorescence studies showed NS5A and HSC70 to colocalize. Unlike HSP70, HSC70 knockdown did not decrease viral protein levels. Rather, intracellular infectious virion assembly was significantly impaired by HSC70 knockdown. We also discovered that both HSC70 nucleotide binding and substrate binding domains directly bind NS5A whereas only the HSP70 nucleotide binding domain does. Knockdown of both HSC70 and HSP70 demonstrated an additive reduction in virus production. This data suggests that HSC70 and HSP70 play discrete roles in the viral life cycle. Investigation of these different functions may facilitate developing of novel strategies that target host proteins to treat HCV infection.

MicroRNAs overexpressed in growth-restricted rat skeletal muscles regulate the glucose transport in cell culture targeting central TGF-ß factor SMAD4.

The micro-array profiling of micro-RNA has been performed in rat skeletal muscle tissues, isolated from male adult offspring of intrauterine plus postnatal growth restricted model (IPGR). Apparently, the GLUT4 mRNA expression in male sk. muscle was found to be unaltered in contrast to females. The over-expression of miR-29a and miR-23a in the experimental group of SMSP (Starved Mother Starved Pups) have been found to regulate the glucose transport activity with respect to their control counterparts CMCP (Control Mother Control Pups) as confirmed in rat L6 myoblast-myocyte cell culture system. The ex-vivo experimentation demonstrates an aberration in insulin signaling pathway in male sk. muscle that leads to the localization of the membrane-bound Glut4 protein. We have identified through a series of experiments one important protein factor SMAD4, a co-SMAD critical to the TGF-beta signaling pathway. This factor is targeted by miR-29a, as identified in an in vitro reporter-assay system in cell-culture experiment. The other micro-RNA, miR-23a, targets SMAD4 indirectly that seems to be critical in regulating insulin-dependent glucose transport activity. MicroRNA mimics, inhibitors and siRNA studies indicate the role of SMAD4 as inhibitory for glucose transport activities in normal physiological condition. The data demonstrate for the first time a critical function of microRNAs in fine-tuning the regulation of glucose transport in skeletal muscle. Chronic starved conditions (IPGR) in sk. muscle up-regulates microRNA changing the target protein expression patterns, such as SMAD4, to alter the glucose transport pathways for the survival. The innovative outcome of this paper identifies a critical pathway (TGF-beta) that may act negatively for the mammalian glucose transport machinery.

Divergent antiviral effects of bioflavonoids on the hepatitis C virus life cycle.

We have previously demonstrated that quercetin, a bioflavonoid, blocks hepatitis C virus (HCV) proliferation by inhibiting NS5A-driven internal ribosomal entry site (IRES)-mediated translation of the viral genome. Here, we investigate the mechanisms of antiviral activity of quercetin and six additional bioflavonoids. We demonstrate that catechin, naringenin, and quercetin possess significant antiviral activity, with no associated cytotoxicity. Infectious virion secretion was not significantly altered by these bioflavonoids. Catechin and naringenin demonstrated stronger inhibition of infectious virion assembly compared to quercetin. Quercetin markedly blocked viral translation whereas catechin and naringenin demonstrated mild activity. Similarly quercetin completely blocked NS5A-augmented IRES-mediated translation in an IRES reporter assay, whereas catechin and naringenin had only a mild effect. Moreover, quercetin differentially inhibited HSP70 induction compared to catechin and naringenin. Thus, the antiviral activity of these bioflavonoids is mediated through different mechanisms. Therefore combination of these bioflavonoids may act synergistically against HCV.

A cell-permeable peptide inhibits hepatitis C virus replication by sequestering IRES transacting factors.

Hepatitis C virus (HCV) infection frequently leads to chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. There is no effective therapy or vaccine available to HCV-infected patients other than interferon-ribavarin combination, which is effective in a relatively small percentage of infected patients. Our previous results have shown that a synthetic peptide (LAP) corresponding to the N-terminal 18 amino acids of the Lupus autoantigen (La) was a potent inhibitor of HCV IRES-mediated translation. We demonstrate here that LAP efficiently blocks HCV replication of infectious JFH1 virus in cell culture. Our data suggest that LAP forms complexes with IRES-transacting factors (ITAFs) PTB and PCBP2. LAP-mediated inhibition of HCV IRES-mediated translation in vitro could be fully rescued by recombinant PCB and PCBP2. Also transient expression of PTB / PCBP2 combination significantly restores HCV replication in LAP-inhibited cultures. These results suggest that ITAFs could be potential targets to block HCV replication.

Activation of ribosomal RNA transcription by hepatitis C virus involves upstream binding factor phosphorylation via induction of cyclin D1.

Hepatitis C virus (HCV) causes chronic infection in humans leading to liver cirrhosis and hepatocellular carcinoma. rRNA transcription, catalyzed by RNA polymerase I (Pol I), plays a critical role in ribosome biogenesis, and changes in Pol I transcription rate are associated with profound alterations in the growth rate of the cell. Because rRNA synthesis is intimately linked to cell growth and frequently up-regulated in many cancers, we hypothesized that HCV might have the ability to activate rRNA synthesis in infected cells. We show here that rRNA promoter-mediated transcription is significantly (10- to 12-fold) activated in human liver-derived cells following infection with type 2 JFH-1 HCV or transfection with the subgenomic type 1 HCV replicon. Further analysis revealed that HCV nonstructural protein 5A (NS5A) was responsible for activation of rRNA transcription. Both the NH(2)-terminal amphipathic helix and the polyproline motifs of NS5A seem to be essential for rRNA transcription activation. The NS5A-dependent activation of rRNA transcription seems to be due to hyperphosphorylation and consequent activation of upstream binding factor (UBF), a Pol I DNA binding transcription factor. We further show that hyperphosphorylation of UBF occurs as a result of up-regulation of both cyclin D1 and cyclin-dependent kinase 4 by the HCV NS5A polypeptide. These results suggest that the endoplasmic reticulum-associated NS5A is able to transduce signals into the nucleoplasm via UBF hyperphosphorylation leading to rRNA transcription activation. These results could, at least in part, explain a mechanism by which HCV contributes to transformation of liver cells.

Histone code modifications repress glucose transporter 4 expression in the intrauterine growth-restricted offspring.

We examined transcriptional and epigenetic mechanism(s) behind diminished skeletal muscle GLUT4 mRNA in intrauterine growth-restricted (IUGR) female rat offspring. An increase in MEF2D (inhibitor) with a decline in MEF2A (activator) and MyoD (co-activator) binding to the glut4 promoter in IUGR versus control was observed. The functional role of MEF2/MyoD-binding sites and neighboring three CpG clusters in glut4 gene transcription was confirmed in C2C12 muscle cells. No differential methylation of these three and other CpG clusters in the glut4 promoter occurred. DNA methyltransferase 1 (DNMT1) in postnatal, DNMT3a, and DNMT3b in adult was differentially recruited with increased MeCP2 (methyl CpG-binding protein) concentrations to bind the IUGR glut4 gene. Covalent modifications of the histone (H) code consisted of H3.K14 de-acetylation by recruitment of histone deacetylase (HDAC) 1 and enhanced association of HDAC4 enzymes. This set the stage for Suv39H1 methylase-mediated di-methylation of H3.K9 and increased recruitment of heterochromatin protein 1alpha, which partially inactivates postnatal and adult IUGR glut4 gene transcription. Further increased interactions in the adult IUGR between DNMT3a/DNMT3b and HDAC1 and MEF2D and HDAC1/HDAC4 and decreased association between MyoD and MEF2A existed. We conclude that epigenetic mechanisms consisting of histone code modifications repress skeletal muscle glut4 transcription in the postnatal period and persist in the adult female IUGR offspring.

Glucose transporter isoform-3 mutations cause early pregnancy loss and fetal growth restriction.

Glucose transporter isoform-3 (GLUT3) is the trophoblastic facilitative glucose transporter. To investigate the role of this isoform in embryonic development, we created a novel GLUT3-null mouse and observed arrested early embryonic development and loss at neurulation stage when both alleles were mutated. This loss occurred despite the presence of other related isoforms, particularly GLUT1. In contrast, when a single allele was mutated, despite increased embryonic cell apoptosis, adaptive changes in the subcellular localization of GLUT3 and GLUT1 in the preimplantation embryo led to postimplantation survival. This survival was compromised by decreased GLUT3-mediated transplacental glucose transport, causing late-gestation fetal growth restriction. This yielded young male and female adults demonstrating catch-up growth, with normal basal glucose, insulin, insulin-like growth factor-I and IGF-binding protein-3 concentrations, fat and lean mass, and glucose and insulin tolerance. We conclude that GLUT3 mutations cause a gene dose-dependent early pregnancy loss or late-gestation fetal growth restriction despite the presence of embryonic and placental GLUT1 and a compensatory increase in system A amino acid placental transport. This critical life-sustaining functional role for GLUT3 in embryonic development provides the basis for investigating the existence of human GLUT3 mutations with similar consequences during early pregnancy.

Zuotin, a DnaJ molecular chaperone, stimulates cap-independent translation in yeast.

A small inhibitor RNA (IRNA) isolated from yeast has previously been shown to efficiently block poliovirus and hepatitis C virus IRES-mediated translation by sequestering mammalian RNA-binding (transacting) factors that play important roles in cap-independent translation. Here we have investigated the IRNA-binding proteins that might be involved in cap-independent translation in the yeast Saccharomyces cerevisiae. We have identified Zuotin, a DnaJ chaperone protein similar to mammalian HSP-40 chaperone, which interacts strongly with IRNA. Using ZUO1-deleted S. cerevisiae, we demonstrate a preferential requirement of Zuo1p for cap-independent translation mediated by the 5' untranslated region of the yeast TFIID mRNA. Further studies using zuo1delta S. cerevisiae complemented with various Zuo1p mutants indicate that the DnaJ domain of Zuo1p, known to influence its interaction with HSP-70, significantly affects cap-independent translation. These results demonstrate for the first time a role for an established chaperone protein in cap-independent translation of a cellular mRNA.

Shutoff of RNA polymerase II transcription by poliovirus involves 3C protease-mediated cleavage of the TATA-binding protein at an alternative site: incomplete shutoff of transcription interferes with efficient viral replication.

The TATA-binding protein (TBP) plays a crucial role in cellular transcription catalyzed by all three DNA-dependent RNA polymerases. Previous studies have shown that TBP is targeted by the poliovirus (PV)-encoded protease 3C(pro) to bring about shutoff of cellular RNA polymerase II-mediated transcription in PV-infected cells. The processing of the majority of viral precursor proteins by 3C(pro) involves cleavages at glutamine-glycine (Q-G) sites. We present evidence that suggests that the transcriptional inactivation of TBP by 3C(pro) involves cleavage at the glutamine 104-serine 105 (Q104-S105) site of TBP and not at the Q18-G19 site as previously thought. The TBP Q104-S105 cleavage by 3C(pro) is greatly influenced by the presence of an aliphatic amino acid at the P4 position, a hallmark of 3C(pro)-mediated proteolysis. To examine the importance of host cell transcription shutoff in the PV life cycle, stable HeLa cell lines were created that express recombinant TBP resistant to cleavage by the viral proteases, called GG rTBP. Transcription shutoff was significantly impaired and delayed in GG rTBP cells upon infection with poliovirus compared with the cells that express wild-type recombinant TBP (wt rTBP). Infection of GG rTBP cells with poliovirus resulted in small plaques, significantly reduced viral RNA synthesis, and lower viral yields compared to the wt rTBP cell line. These results suggest that a defect in transcription shutoff can lead to inefficient replication of poliovirus in cultured cells.

Nuclear entry of poliovirus protease-polymerase precursor 3CD: implications for host cell transcription shut-off.

Host cell transcription mediated by all three RNA polymerases is rapidly inhibited after infection of mammalian cells with poliovirus (PV). Both genetic and biochemical studies have shown that the virus-encoded protease 3C cleaves the TATA-binding protein and other transcription factors at glutamine-glycine sites and is directly responsible for host cell transcription shut-off. PV replicates in the cytoplasm of infected cells. To shut-off host cell transcription, 3C or a precursor of 3C must enter the nucleus of infected cells. Although the 3C protease itself lacks a nuclear localization signal (NLS), amino acid sequence examination of 3D identified a potential single basic type NLS, KKKRD, spanning amino acids 125-129 within this polypeptide. Thus, a plausible scenario is that 3C enters the nucleus in the form of its precursor, 3CD, which then generates 3C by auto-proteolysis ultimately leading to cleavage of transcription factors in the nucleus. Using transient transfection of enhanced green fluorescent protein (EGFP) fusion polypeptides, we demonstrate here that both 3CD and 3D are capable of entering the nucleus in PV-infected cells. However, both polypeptides remain in the cytoplasm in uninfected HeLa cells. Mutagenesis of the NLS sequence in 3D prevents nuclear entry of 3D and 3CD in PV-infected cells. We also demonstrate that 3CD can be detected in the nuclear fraction from PV-infected HeLa cells as early as 2 h postinfection. Significant amount of 3CD is found associated with the nuclear fraction by 3-4 h of infection. Taken together, these results suggest that both the 3D NLS and PV infection are required for the entry of 3CD into the nucleus and that this may constitute a means by which viral protease 3C is delivered into the nucleus leading to host cell transcription shut-off.

Glutamate and cyclic AMP regulate the expression of galactokinase in Mycobacterium smegmatis.

It was found that Mycobacterium smegmatis is unable to utilize galactose as the sole carbon source because the sugar alone cannot induce galactokinase. However, galactokinase was induced by glutamate alone, and was further stimulated by galactose. Rifampicin completely inhibited the glutamate-mediated expression of galK in both the absence and presence of galactose. Extracellular cAMP stimulated the expression of the enzyme only in the presence of glutamate plus galactose. The galK gene from M. smegmatis, including its upstream promoter region, was cloned in a plasmid in Escherichia coli. The expression of kinase from these clones in E. coli was dependent on cAMP and its receptor protein (CRP). The expression of UDP-galactose 4-epimerase was constitutive. This and other evidence suggests that the galK gene is not linked to galT and galE in the mycobacterial genome. In a glutamate-independent galactose-utilizing mutant (gin-1 mutant) of M. smegmatis, galK was expressed in the absence of both galactose and glutamate, while in the presence of galactose this expression was increased twofold in the absence of glutamate and fourfold in its presence. Extracellularly added cAMP reduced the expression of the enzyme in the presence of galactose plus glutamate nearly to the basal level. It is proposed that in M. smegmatis the galK gene is expressed from two different promoters; the expression from one promoter is dependent on glutamate but not on galactose and cAMP, while that from the other requires all three components. The role of galactose is possibly to derepress the latter promoter.

Primer utilization by DNA polymerase alpha-primase is influenced by its interaction with Mcm10p.

Models of DNA replication in yeast and Xenopus suggest that Mcm10p is required to generate the pre-initiation complex as well as progression of the replication fork during the elongation of DNA chains. In this report, we show that the Schizosaccharomyces pombe Mcm10p/Cdc23p binds to the S. pombe DNA polymerase (pol) alpha-primase complex in vitro by interacting specifically with the catalytic p180 subunit and stimulates DNA synthesis catalyzed by the pol alpha-primase complex with various primed DNA templates. We investigated the mechanism by which Mcm10p activates the polymerase activity of the pol alpha-primase complex by generating truncated derivatives of the full-length 593-amino acid Mcm10p. Their ability to stimulate pol alpha polymerase activity and bind to single-stranded DNA and to pol alpha were compared. Concomitant with increased deletion of the N-terminal region (from amino acids 95 to 415), Mcm10p derivatives lost their ability to stimulate pol alpha polymerase activity and bind to single-stranded DNA. Truncated derivatives of Mcm10p containing amino acids 1-416 retained the pol alpha binding activity, whereas the C terminus, amino acids 496-593, did not. These results demonstrate that both the single-stranded DNA binding and the pol alpha binding properties of Mcm10p play important roles in the activation. In accord with these findings, Mcm10p facilitated the binding of pol alpha-primase complex to primed DNA and formed a stable complex with pol alpha-primase on primed templates. A mutant that failed to activate or bind to DNA and pol alpha, was not observed in this complex. We suggest that the interaction of Mcm10p with the pol alpha-primase complex, its binding to single-stranded DNA, and its activation of the polymerase complex together contribute to its role in the elongation phase of DNA replication.

A peptide from autoantigen La blocks poliovirus and hepatitis C virus cap-independent translation and reveals a single tyrosine critical for La RNA binding and translation stimulation.

La, a 52-kDa autoantigen in patients with systemic lupus erythematosus, was one of the first cellular proteins identified to interact with viral internal ribosome entry site (IRES) elements and stimulate poliovirus (PV) and hepatitis C virus (HCV) IRES-mediated translation. Previous results from our laboratory have shown that a small, yeast RNA (IRNA) could selectively inhibit PV and HCV IRES-mediated translation by sequestering the La protein. Here we have identified an 18-amino-acid-long sequence from the N-terminal "La motif" which is required for efficient interaction of La with IRNA and viral 5' untranslated region (5'-UTR) elements. A synthetic peptide (called LAP, for La peptide) corresponding to this sequence (amino acids 11 to 28) of La was found to efficiently inhibit viral IRES-mediated translation in vitro. The LAP efficiently enters Huh-7 cells and preferentially inhibits HCV IRES-mediated translation programmed by a bicistronic RNA in vivo. The LAP does not bind RNA directly but appears to block La binding to IRNA and PV 5'-UTR. Competition UV cross-link and translation rescue experiments suggested that LAP inhibits IRES-mediated translation by interacting with proteins rather than RNA. Mutagenesis of LAP demonstrates that single amino acid changes in a highly conserved sequence within LAP are sufficient to eliminate the translation-inhibitory activity of LAP. When one of these mutations (Y23Q) is introduced into full-length La, the mutant protein is severely defective in interacting with the PV IRES element and consequently unable to stimulate IRES-mediated translation. However, the La protein with a mutation of the next tyrosine moiety (Y24Q) could still interact with PV 5'-UTR and stimulate viral IRES-mediated translation significantly. These results underscore the importance of the La N-terminal amino acids in RNA binding and viral RNA translation. The possible role of the LAP sequence in La-RNA binding and stimulation of viral IRES-mediated translation is discussed.