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==prototype foamy virus strand transfer complexes on product DNA==
<StructureSection load='4bac' size='340' side='right'caption='[[4bac]], [[Resolution|resolution]] 3.26&Aring;' scene=''>
== Structural highlights ==
<table><tr><td colspan='2'>[[4bac]] is a 5 chain structure with sequence from [http://en.wikipedia.org/wiki/Foamv Foamv]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=4BAC OCA]. For a <b>guided tour on the structure components</b> use [http://oca.weizmann.ac.il/oca-docs/fgij/fg.htm?mol=4BAC FirstGlance]. <br>
</td></tr><tr id='ligand'><td class="sblockLbl"><b>[[Ligand|Ligands:]]</b></td><td class="sblockDat"><scene name='pdbligand=MG:MAGNESIUM+ION'>MG</scene>, <scene name='pdbligand=ZN:ZINC+ION'>ZN</scene></td></tr>
<tr id='related'><td class="sblockLbl"><b>[[Related_structure|Related:]]</b></td><td class="sblockDat">[[2x6n|2x6n]], [[2x6s|2x6s]], [[2x74|2x74]], [[2x78|2x78]]</td></tr>
<tr id='resources'><td class="sblockLbl"><b>Resources:</b></td><td class="sblockDat"><span class='plainlinks'>[http://oca.weizmann.ac.il/oca-docs/fgij/fg.htm?mol=4bac FirstGlance], [http://oca.weizmann.ac.il/oca-bin/ocaids?id=4bac OCA], [http://pdbe.org/4bac PDBe], [http://www.rcsb.org/pdb/explore.do?structureId=4bac RCSB], [http://www.ebi.ac.uk/pdbsum/4bac PDBsum], [http://prosat.h-its.org/prosat/prosatexe?pdbcode=4bac ProSAT]</span></td></tr>
</table>
== Function ==
[[http://www.uniprot.org/uniprot/POL_FOAMV POL_FOAMV]] The aspartyl protease activity mediates proteolytic cleavages of Gag and Pol polyproteins. The reverse transcriptase (RT) activity converts the viral RNA genome into dsDNA in the cytoplasm, shortly after virus entry into the cell (early reverse transcription) or after proviral DNA transcription (late reverse transcription). RT consists of a DNA polymerase activity that can copy either DNA or RNA templates, and a ribonuclease H (RNase H) activity that cleaves the RNA strand of RNA-DNA heteroduplexes in a partially processive 3' to 5' endonucleasic mode. Conversion of viral genomic RNA into dsDNA requires many steps. A tRNA-Lys1,2 binds to the primer-binding site (PBS) situated at the 5'-end of the viral RNA. RT uses the 3' end of the tRNA primer to perform a short round of RNA-dependent minus-strand DNA synthesis. The reading proceeds through the U5 region and ends after the repeated (R) region which is present at both ends of viral RNA. The portion of the RNA-DNA heteroduplex is digested by the RNase H, resulting in a ssDNA product attached to the tRNA primer. This ssDNA/tRNA hybridizes with the identical R region situated at the 3' end of viral RNA. This template exchange, known as minus-strand DNA strong stop transfer, can be either intra- or intermolecular. RT uses the 3' end of this newly synthesized short ssDNA to perform the RNA-dependent minus-strand DNA synthesis of the whole template. RNase H digests the RNA template except for a polypurine tract (PPT) situated at the 5'-end and near the center of the genome. It is not clear if both polymerase and RNase H activities are simultaneous. RNase H probably can proceed both in a polymerase-dependent (RNA cut into small fragments by the same RT performing DNA synthesis) and a polymerase-independent mode (cleavage of remaining RNA fragments by free RTs). Secondly, RT performs DNA-directed plus-strand DNA synthesis using the PPT that has not been removed by RNase H as primer. PPT and tRNA primers are then removed by RNase H. The 3' and 5' ssDNA PBS regions hybridize to form a circular dsDNA intermediate. Strand displacement synthesis by RT to the PBS and PPT ends produces a blunt ended, linear dsDNA copy of the viral genome that includes long terminal repeats (LTRs) at both ends (By similarity).  Integrase catalyzes viral DNA integration into the host chromosome, by performing a series of DNA cutting and joining reactions. This enzyme activity takes place after virion entry into a cell and reverse transcription of the RNA genome in dsDNA. The first step in the integration process is 3' processing. This step requires a complex comprising at least the viral genome, matrix protein, and integrase. This complex is called the pre-integration complex (PIC). The integrase protein removes 2 nucleotides from the 3' end of the viral DNA right (U5) end, leaving the left (U3) intact. In the second step, the PIC enters cell nucleus. This process is mediated through the integrase and allows the virus to infect both dividing (nuclear membrane disassembled) and G1/S-arrested cells (active translocation), but with no viral gene expression in the latter. In the third step, termed strand transfer, the integrase protein joins the previously processed 3' ends to the 5' ends of strands of target cellular DNA at the site of integration. It is however not clear how integration then proceeds to resolve the asymmetrical cleavage of viral DNA (By similarity). 
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== Publication Abstract from PubMed ==
Integrase is the key enzyme that mediates integration of retroviral DNA into cellular DNA which is essential for viral replication. Inhibitors of HIV-1 that target integrase recognize the nucleoprotein complexes formed by integrase and viral DNA substrate (intasomes) rather than the free enzyme. Atomic resolution structures of HIV-1 intasomes are therefore required to understand the mechanisms of inhibition and drug resistance. To date, prototype foamy virus (PFV) is the only retrovirus for which such structures have been determined. We show that PFV Strand Transfer Complexes (STC) can be assembled on product DNA without going through the normal forward reaction pathway. The finding that a retroviral STC can be assembled in this way may provide a powerful tool to alleviate the obstacles that impede structural studies of nucleoprotein intermediates in HIV-1 DNA integration.

Assembly of prototype foamy virus strand transfer complexes on product DNA bypassing catalysis of integration.,Yin Z, Lapkouski M, Yang W, Craigie R Protein Sci. 2012 Sep 25. doi: 10.1002/pro.2166. PMID:23011895<ref>PMID:23011895</ref>

From MEDLINE&reg;/PubMed&reg;, a database of the U.S. National Library of Medicine.<br>
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<div class="pdbe-citations 4bac" style="background-color:#fffaf0;"></div>
== References ==
<references/>
__TOC__
</StructureSection>
[[Category: Foamv]]
[[Category: Large Structures]]
[[Category: Craigie, R]]
[[Category: Lapkouski, M]]
[[Category: Yang, W]]
[[Category: Yin, Z]]
[[Category: Transferase-dna complex]]