Retroviral Gag proteins are targeted to the plasma membrane, where they play the central role in virion formation. Several studies have suggested that the membrane-binding signal is contained within the amino-terminal matrix sequence; however, the precise location has never been determined for the Gag protein of any retrovirus. In this report, we show that the first 31 residues of human immunodeficiency virus type 1 Gag protein can function independently as a membrane-targeting domain when fused to heterologous proteins. A bipartite membrane-targeting motif was identified, consisting of the myristylated N-terminal 14 amino acids and a highly basic region that binds acidic phospholipids. Replacement of the N-terminal membrane-targeting domain of pp6o-src with that of human immunodeficiency virus type 1 Gag elicits efficient membrane binding and a transforming phenotype. Removal of myristate or the basic region results in decreased membrane binding of Gag-Src chimeras in vitro and impaired virion formation by Pr55rag in vivo. We propose that the N-terminal Gag sequence functions as a targeting signal to direct interaction with acidic phospholipids on the cytoplasmic leaflet of the plasma membrane.
The retroviral Gag polyprotein directs budding from the plasma membrane of infected cells. Until now, it was believed that Gag proteins of type C retroviruses, including the prototypic oncoretrovirus Rous sarcoma virus, were synthesized on cytosolic ribosomes and targeted directly to the plasma membrane. Here we reveal a previously unknown step in the subcellular trafficking of the Gag protein, that of transient nuclear localization. We have identified a targeting signal within the N-terminal matrix domain that facilitates active nuclear import of the Gag polyprotein. We also found that Gag is transported out of the nucleus through the CRM1 nuclear export pathway, based on observations that treatment of virus-expressing cells with leptomycin B resulted in the redistribution of Gag proteins from the cytoplasm to the nucleus. Internal deletion of the C-terminal portion of the Gag p10 region resulted in the nuclear sequestration of Gag and markedly diminished budding, suggesting that the nuclear export signal might reside within p10. Finally, we observed that a previously described matrix mutant, Myr1E, was insensitive to the effects of leptomycin B, apparently bypassing the nuclear compartment during virus assembly. Myr1E has a defect in genomic RNA packaging, implying that nuclear localization of Gag might be involved in viral RNA interactions. Taken together, these findings provide evidence that nuclear entry and egress of the Gag polyprotein are intrinsic components of the Rous sarcoma virus assembly pathway. Retroviruses must gain access to the nucleus to replicate. After receptor binding, entry, and reverse transcription, the integration-competent nucleoprotein complex (called the preintegration complex or PIC) enters the nucleus. For oncoretroviruses like Rous sarcoma virus (RSV) that primarily infect dividing cells, the PIC awaits breakdown of the nuclear envelope during mitosis for nuclear entry. Lentiviruses including HIV-1 infect nondividing cells, and PICs are transported through intact nuclear envelopes. HIV-1 nuclear entry is complex, and redundant signals have been identified in the viral matrix (MA), integrase, and Vpr proteins (reviewed in ref. 1). The recent report that RSV can replicate at low levels in quiescent cells does raise the possibility that a viral protein might mediate active nuclear targeting of the RSV PIC (2).After nuclear entry of the PIC and proviral integration, viral RNA is transcribed, and unspliced genome-length viral mRNAs must exit the nucleus for translation into viral structural proteins and encapsidation into virions. The nuclear export of introncontaining mRNAs is normally inhibited by cellular mechanisms, so retroviruses must circumvent this obstacle. Lentiviruses encode trans-acting factors such as the HIV-1 Rev protein to mediate nuclear export of intron-containing viral RNAs (3). Oncoretroviruses including RSV lack Rev-like transport factors and instead have cis-acting constitutive transport elements to facilitate the export of unspliced viral RNA (4).The regulation ...
Despite the vast excess of cellular RNAs, precisely two copies of viral genomic RNA (gRNA) are selectively packaged into new human immunodeficiency type 1 (HIV-1) particles via specific interactions between the HIV-1 Gag and the gRNA psi (ψ) packaging signal. Gag consists of the matrix (MA), capsid, nucleocapsid (NC), and p6 domains. Binding of the Gag NC domain to ψ is necessary for gRNA packaging, but the mechanism by which Gag selectively interacts with ψ is unclear. Here, we investigate the binding of NC and Gag variants to an RNA derived from ψ (Psi RNA), as well as to a non-ψ region (TARPolyA). Binding was measured as a function of salt to obtain the effective charge (Z eff ) and nonelectrostatic (i.e., specific) component of binding, K d(1M) . Gag binds to Psi RNA with a dramatically reduced K d(1M) and lower Z eff relative to TARPolyA. NC, GagΔMA, and a dimerization mutant of Gag bind TARPolyA with reduced Z eff relative to WT Gag. Mutations involving the NC zinc finger motifs of Gag or changes to the G-rich NC-binding regions of Psi RNA significantly reduce the nonelectrostatic component of binding, leading to an increase in Z eff . These results show that Gag interacts with gRNA using different binding modes; both the NC and MA domains are bound to RNA in the case of TARPolyA, whereas binding to Psi RNA involves only the NC domain. Taken together, these results suggest a novel mechanism for selective gRNA encapsidation.
The Gag proteins of Rous sarcoma virus and human immunodeficiency virus (HIV) each contain a function involved in a late step in budding, defects in which result in the accumulation of these molecules at the plasma membrane. In the Rous sarcoma virus Gag protein (Pr76 gag), this assembly domain is associated with a PPPY motif, which is located at an internal position between the MA and CA sequences. This motif is not contained anywhere within the HIV Gag protein (Pr55 gag), and the MA sequence is linked directly to CA. Instead, a late assembly function of HIV has been associated with the p6 sequence situated at the C terminus of Gag. Here we demonstrate the remarkable finding that the late assembly domains from these two unrelated Gag proteins are exchangeable between retroviruses and can function in a positionally independent manner.
Abstract. We present durational data on normal oral-pharyngeal swallows in adults obtained using ultrasound imaging, The effects of normal aging on the oral-pharyngeal phase of swallowing were studied in 47 healthy adults. Timing of the oralpharyngeal phase of swallow was determined from frame-by-flame analysis of ultrasound videos of the motion of the tongue and hyoid bone from initial rest to final resting position. Duration of unstimulated (dry) swallows was compared to stimulated (wet) swallows across four age groups and by sex and age. For most subjects, dry swallows were longer than wet swallows; moreover, swallow duration was longest for older women than any other group. As age increased (55+), oral swallows were accompanied by extralingual gestures. Ability to produce a timed series of continuously dry swallows was somewhat influenced by age. Findings are suggestive of an age change more typical in women, with a pattern of multiple lingual gestures commonly seen after age 55 in both sexes. We suggest that subtle, subclinical, oral neuromotor changes occur with normal aging to cause these findings.
We analyzed the emergence of daptomycin nonsusceptibility in a patient with persistent vancomycin-intermediate Staphylococcus aureus (VISA) bacteremia. The daptomycin-nonsusceptible VISA's cell wall demonstrated a reduction in muramic acid O-acetylation, a phenotypic parameter not previously reported for VISA; some isolates also contained a single point mutation in the mprF gene.
Retroviral Gag polyproteins coopt host factors to traffic from cytosolic ribosomes to the plasma membrane, where virions are released. Before membrane transport, the multidomain Gag protein of Rous sarcoma virus (RSV) undergoes importin-mediated nuclear import and CRM1-dependent nuclear export, an intrinsic step in the assembly pathway. Transient nuclear trafficking of Gag is required for efficient viral RNA (vRNA) encapsidation, suggesting that Gag: vRNA binding might occur in the nucleus. Here, we show that Gag is imported into the nucleus through direct interactions of the Gag NC domain with importin-α (imp-α) and the MA domain with importin-11 (imp-11). The vRNA packaging signal, known as ψ, inhibited imp-α binding to Gag, indicating that the NC domain does not bind to imp-α and vRNA simultaneously. Unexpectedly, vRNA binding also prevented the association of imp-11 with both the MA domain alone and with Gag, suggesting that the MA domain may bind to the vRNA genome. In contrast, direct binding of Gag to the nuclear export factor CRM1, via the CRM1-RanGTP heterodimer, was stimulated by ψRNA. These findings suggest a model whereby the genomic vRNA serves as a switch to regulate the ordered association of host import/export factors that mediate Gag nucleocytoplasmic trafficking for virion assembly. The Gag:vRNA interaction appears to serve multiple critical roles in assembly: specific selection of the vRNA genome for packaging, stimulating the formation of Gag dimers, and triggering export of viral ribonucleoprotein complexes from the nucleus.protein-RNA binding | retrovirus assembly | importin-α/β | importin-11 | CRM1
The packaging of retroviral genomic RNA (gRNA) requires cis-acting elements within the RNA and transacting elements within the Gag polyprotein. The packaging signal , at the 5 end of the viral gRNA, binds to Gag through interactions with basic residues and Cys-His box RNA-binding motifs in the nucleocapsid. Although specific interactions between Gag and gRNA have been demonstrated previously, where and when they occur is not well understood. We discovered that the Rous sarcoma virus (RSV) Gag protein transiently localizes to the nucleus, although the roles of Gag nuclear trafficking in virus replication have not been fully elucidated. A mutant of RSV (Myr1E) with enhanced plasma membrane targeting of Gag fails to undergo nuclear trafficking and also incorporates reduced levels of gRNA into virus particles compared to those in wild-type particles. Based on these results, we hypothesized that Gag nuclear entry might facilitate gRNA packaging. To test this idea by using a gain-of-function genetic approach, a bipartite nuclear localization signal (NLS) derived from the nucleoplasmin protein was inserted into the Myr1E Gag sequence (generating mutant Myr1E.NLS) in an attempt to restore nuclear trafficking. Here, we report that the inserted NLS enhanced the nuclear localization of Myr1E.NLS Gag compared to that of Myr1E Gag. Also, the NLS sequence restored gRNA packaging to nearly wild-type levels in viruses containing Myr1E.NLS Gag, providing genetic evidence linking nuclear trafficking of the retroviral Gag protein with gRNA incorporation.The encapsidation of the RNA genome is essential for retrovirus replication. Because the viral genomic RNA (gRNA) constitutes only a small fraction of the total cellular mRNA, a specific Gag-RNA interaction is thought to be required for viral genome packaging (2). The determinants of virus-specific gRNA incorporation include the cis-acting element at the 5Јend of the viral gRNA, known as the packaging signal (), and the nucleocapsid (NC) domain of the Gag polyprotein (3, 14, 62). In Rous sarcoma virus (RSV), the NC domain contains basic residues that are required for the recognition of and binding to , as well as two Cys-His motifs that maintain the overall conformation of NC and are essential for RNA packaging (30, 31).Packaging of gRNA into progeny virions requires that the unspliced viral mRNA be exported from the nucleus. However, cellular proofreading mechanisms ensure that unspliced or intron-containing mRNAs are retained in the nucleus until splicing occurs. Complex retroviruses like human immunodeficiency virus type 1 (HIV-1) overcome this export block of unspliced genomes by encoding the Rev protein, which interacts with a cis-acting sequence in the viral RNA (the Revresponsive element [RRE]) to facilitate cytoplasmic accumulation of intron-containing viral mRNA (16,35). The export of the Rev-viral RNA complex is mediated through the interaction of a leucine-rich nuclear export signal (NES) in Rev with the CRM1 nuclear export factor (17,18,37,41). Simple retroviruses do no...
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