RESUMO
Vertebrate genomes exhibit marked CG suppression-that is, lower than expected numbers of 5'-CG-3' dinucleotides. This feature is likely to be due to C-to-T mutations that have accumulated over hundreds of millions of years, driven by CG-specific DNA methyl transferases and spontaneous methyl-cytosine deamination. Many RNA viruses of vertebrates that are not substrates for DNA methyl transferases mimic the CG suppression of their hosts. This property of viral genomes is unexplained. Here we show, using synonymous mutagenesis, that CG suppression is essential for HIV-1 replication. The deleterious effect of CG dinucleotides on HIV-1 replication was cumulative, associated with cytoplasmic RNA depletion, and was exerted by CG dinucleotides in both translated and non-translated exonic RNA sequences. A focused screen using small inhibitory RNAs revealed that zinc-finger antiviral protein (ZAP) inhibited virion production by cells infected with CG-enriched HIV-1. Crucially, HIV-1 mutants containing segments whose CG content mimicked random nucleotide sequence were defective in unmanipulated cells, but replicated normally in ZAP-deficient cells. Crosslinking-immunoprecipitation-sequencing assays demonstrated that ZAP binds directly and selectively to RNA sequences containing CG dinucleotides. These findings suggest that ZAP exploits host CG suppression to identify non-self RNA. The dinucleotide composition of HIV-1, and perhaps other RNA viruses, appears to have adapted to evade this host defence.
Assuntos
Fosfatos de Dinucleosídeos/genética , Sequência Rica em GC/genética , HIV-1/genética , HIV-1/imunologia , RNA Viral/genética , RNA Viral/imunologia , Linhagem Celular , Citoplasma/genética , Citoplasma/virologia , HIV-1/crescimento & desenvolvimento , Humanos , Imunoprecipitação , Mutagênese , Mutação , Ligação Proteica , RNA Viral/metabolismo , Proteínas de Ligação a RNA/genética , Proteínas de Ligação a RNA/metabolismo , Replicação Viral/genéticaRESUMO
Because of evolutionary pressures imposed through episodic colonization by retroviruses, many mammals express factors, such as TRIM5alpha and APOBEC3 proteins, that directly restrict retroviral replication. TRIM5 and APOBEC restriction factors are most often studied in the context of modern primate lentiviruses, but it is likely that ancient retroviruses imposed the selective pressure that is evident in primate TRIM5 and APOBEC3 genes. Moreover, these antiretroviral factors have been shown to act against a variety of retroviruses, including gammaretroviruses. Endogenous retroviruses can provide a 'fossil record' of extinct retroviruses and perhaps evidence of ancient TRIM5 and APOBEC3 antiviral activity. Here, we investigate whether TRIM5 and APOBEC3 proteins restricted the replication of two groups of gammaretroviruses that were endogenized in the past few million years. These endogenous retroviruses appear quite widespread in the genomes of old world primates but failed to colonize the human germline. Our analyses suggest that TRIM5alpha proteins did not pose a major barrier to the cross-species transmission of these two families of gammaretroviruses, and did not contribute to their extinction. However, we uncovered extensive evidence for inactivation of ancient gammaretroviruses through the action of APOBEC3 cytidine deaminases. Interestingly, the identities of the cytidine deaminases responsible for inactivation appear to have varied in both a virus and host species-dependent manner. Overall, sequence analyses and reconstitution of ancient retroviruses from remnants that have been preserved in the genomes of modern organisms offer the opportunity to probe and potentially explain the evolutionary history of host defenses against retroviruses.
Assuntos
Citidina Desaminase/fisiologia , Retrovirus Endógenos/fisiologia , Gammaretrovirus/fisiologia , Primatas/virologia , Proteínas/fisiologia , Replicação Viral , Animais , Proteínas do Capsídeo/química , Proteínas do Capsídeo/genética , Citidina Desaminase/genética , Evolução Molecular , Interações Hospedeiro-Patógeno/genética , Humanos , Macaca mulatta/genética , Macaca mulatta/fisiologia , Macaca mulatta/virologia , Camundongos/genética , Camundongos/fisiologia , Camundongos/virologia , Modelos Moleculares , Células NIH 3T3 , Pan troglodytes/genética , Pan troglodytes/fisiologia , Pan troglodytes/virologia , Filogenia , Primatas/genética , Proteínas/genética , Ubiquitina-Proteína Ligases , Replicação Viral/genéticaRESUMO
Using a yeast two-hybrid screen, we found that SNIP1 (Smad nuclear-interacting protein 1) associates with c-Myc, a key regulator of cell proliferation and transformation. We demonstrate that SNIP1 functions as an important regulator of c-Myc activity, binding the N terminus of c-Myc through its own C terminus, and that SNIP1 enhances the transcriptional activity of c-Myc both by stabilizing it against proteosomal degradation and by bridging the c-Myc/p300 complex. These effects of SNIP1 on c-Myc likely contribute to synergistic effects of SNIP1, c-Myc, and H-Ras in inducing formation of foci in an in vitro transformation assay and also in supporting anchorage-independent growth. The significant association of SNIP1 and c-Myc staining in a non-small cell lung cancer tissue array is further evidence that their activities might be linked and suggests that SNIP1 might be an important modulator of c-Myc activity in carcinogenesis.