| Structural highlights
1t05 is a 4 chain structure with sequence from 9hiv1. Full crystallographic information is available from OCA. For a guided tour on the structure components use FirstGlance.
| | Ligands: | , , |
| NonStd Res: | , |
| Related: | 1rtd, 1t03 |
| Gene: | POL (9HIV1) |
| Activity: | RNA-directed DNA polymerase, with EC number 2.7.7.49 |
| Resources: | FirstGlance, OCA, PDBe, RCSB, PDBsum, ProSAT |
Function
[POL_HV1H2] Gag-Pol polyprotein and Gag polyprotein may regulate their own translation, by the binding genomic RNA in the 5'-UTR. At low concentration, Gag-Pol and Gag would promote translation, whereas at high concentration, the polyproteins encapsidate genomic RNA and then shutt off translation.[1] [2] [3] [4] Matrix protein p17 has two main functions: in infected cell, it targets Gag and Gag-pol polyproteins to the plasma membrane via a multipartite membrane-binding signal, that includes its myristoylated N-terminus. The second function is to play a role in nuclear localization of the viral genome at the very start of cell infection. Matrix protein is the part of the pre-integration complex. It binds in the cytoplasm the human BAF protein which prevent autointegration of the viral genome, and might be included in virions at the ration of zero to 3 BAF dimer per virion. The myristoylation signal and the NLS thus exert conflicting influences its subcellular localization. The key regulation of these motifs might be phosphorylation of a portion of MA molecules on the C-terminal tyrosine at the time of virus maturation, by virion-associated cellular tyrosine kinase. Implicated in the release from host cell mediated by Vpu.[5] [6] [7] [8] Capsid protein p24 forms the conical core that encapsulates the genomic RNA-nucleocapsid complex in the virion. Most core are conical, with only 7% tubular. The core is constituted by capsid protein hexamer subunits. The core is disassembled soon after virion entry. Interaction with human PPIA/CYPA protects the virus from restriction by human TRIM5-alpha and from an unknown antiviral activity in human cells. This capsid restriction by TRIM5 is one of the factors which restricts HIV-1 to the human species.[9] [10] [11] [12] Nucleocapsid protein p7 encapsulates and protects viral dimeric unspliced (genomic) RNA. Binds these RNAs through its zinc fingers. Facilitates rearangement of nucleic acid secondary structure during retrotranscription of genomic RNA. This capability is referred to as nucleic acid chaperone activity.[13] [14] [15] [16] The aspartyl protease mediates proteolytic cleavages of Gag and Gag-Pol polyproteins during or shortly after the release of the virion from the plasma membrane. Cleavages take place as an ordered, step-wise cascade to yield mature proteins. This process is called maturation. Displays maximal activity during the budding process just prior to particle release from the cell. Also cleaves Nef and Vif, probably concomitantly with viral structural proteins on maturation of virus particles (By similarity).[17] [18] [19] [20] Reverse transcriptase/ribonuclease H (RT) is a multifunctional enzyme that converts the viral RNA genome into dsDNA in the cytoplasm, shortly after virus entry into the cell. This enzyme displays 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(3)-Lys 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 two polypurine tracts (PPTs) 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 PPTs that have not been removed by RNase H as primers. PPTs 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.[21] [22] [23] [24] 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 the viral genome, matrix protein, Vpr and integrase. This complex is called the pre-integration complex (PIC). The integrase protein removes 2 nucleotides from each 3' end of the viral DNA, leaving recessed CA OH's at the 3' ends. In the second step, the PIC enters cell nucleus. This process is mediated through integrase and Vpr proteins, and allows the virus to infect a non dividing cell. This ability to enter the nucleus is specific of lentiviruses, other retroviruses cannot and rely on cell division to access cell chromosomes. 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. The 5'-ends are produced by integrase-catalyzed staggered cuts, 5 bp apart. A Y-shaped, gapped, recombination intermediate results, with the 5'-ends of the viral DNA strands and the 3' ends of target DNA strands remaining unjoined, flanking a gap of 5 bp. The last step is viral DNA integration into host chromosome. This involves host DNA repair synthesis in which the 5 bp gaps between the unjoined strands are filled in and then ligated. Since this process occurs at both cuts flanking the HIV genome, a 5 bp duplication of host DNA is produced at the ends of HIV-1 integration. Alternatively, Integrase may catalyze the excision of viral DNA just after strand transfer, this is termed disintegration.[25] [26] [27] [28] [POL_HV1B1] Gag-Pol polyprotein and Gag polyprotein may regulate their own translation, by the binding genomic RNA in the 5'-UTR. At low concentration, Gag-Pol and Gag would promote translation, whereas at high concentration, the polyproteins encapsidate genomic RNA and then shutt off translation (By similarity).[29] Matrix protein p17 has two main functions: in infected cell, it targets Gag and Gag-pol polyproteins to the plasma membrane via a multipartite membrane-binding signal, that includes its myristoylated N-terminus. The second function is to play a role in nuclear localization of the viral genome at the very start of cell infection. Matrix protein is the part of the pre-integration complex. It binds in the cytoplasm the human BAF protein which prevent autointegration of the viral genome, and might be included in virions at the ration of zero to 3 BAF dimer per virion. The myristoylation signal and the NLS thus exert conflicting influences its subcellular localization. The key regulation of these motifs might be phosphorylation of a portion of MA molecules on the C-terminal tyrosine at the time of virus maturation, by virion-associated cellular tyrosine kinase. Implicated in the release from host cell mediated by Vpu (By similarity).[30] Capsid protein p24 forms the conical core that encapsulates the genomic RNA-nucleocapsid complex in the virion. Most core are conical, with only 7% tubular. The core is constituted by capsid protein hexamer subunits. The core is disassembled soon after virion entry. Interaction with human PPIA/CYPA protects the virus from restriction by human TRIM5-alpha and from an unknown antiviral activity in human cells. This capsid restriction by TRIM5 is one of the factors which restricts HIV-1 to the human species (By similarity).[31] Nucleocapsid protein p7 encapsulates and protects viral dimeric unspliced (genomic) RNA. Binds these RNAs through its zinc fingers. Facilitates rearangement of nucleic acid secondary structure during retrotranscription of genomic RNA. This capability is referred to as nucleic acid chaperone activity (By similarity).[32] The aspartyl protease mediates proteolytic cleavages of Gag and Gag-Pol polyproteins during or shortly after the release of the virion from the plasma membrane. Cleavages take place as an ordered, step-wise cascade to yield mature proteins. This process is called maturation. Displays maximal activity during the budding process just prior to particle release from the cell. Also cleaves Nef and Vif, probably concomitantly with viral structural proteins on maturation of virus particles (By similarity).[33] Reverse transcriptase/ribonuclease H (RT) is a multifunctional enzyme that converts the viral RNA genome into dsDNA in the cytoplasm, shortly after virus entry into the cell. This enzyme displays 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(3)-Lys 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 two polypurine tracts (PPTs) 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 PPTs that have not been removed by RNase H as primers. PPTs 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).[34] 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 the viral genome, matrix protein, Vpr and integrase. This complex is called the pre-integration complex (PIC). The integrase protein removes 2 nucleotides from each 3' end of the viral DNA, leaving recessed CA OH's at the 3' ends. In the second step, the PIC enters cell nucleus. This process is mediated through integrase and Vpr proteins, and allows the virus to infect a non dividing cell. This ability to enter the nucleus is specific of lentiviruses, other retroviruses cannot and rely on cell division to access cell chromosomes. 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. The 5'-ends are produced by integrase-catalyzed staggered cuts, 5 bp apart. A Y-shaped, gapped, recombination intermediate results, with the 5'-ends of the viral DNA strands and the 3' ends of target DNA strands remaining unjoined, flanking a gap of 5 bp. The last step is viral DNA integration into host chromosome. This involves host DNA repair synthesis in which the 5 bp gaps between the unjoined strands are filled in and then ligated. Since this process occurs at both cuts flanking the HIV genome, a 5 bp duplication of host DNA is produced at the ends of HIV-1 integration. Alternatively, Integrase may catalyze the excision of viral DNA just after strand transfer, this is termed disintegration (By similarity).[35]
Evolutionary Conservation
Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf.
Publication Abstract from PubMed
Tenofovir, also known as PMPA, R-9-(2-(phosphonomethoxypropyl)adenine, is a nucleotide reverse transcriptase (RT) inhibitor. We have determined the crystal structures of two related complexes of HIV-1 RT with template primer and tenofovir: (i) a ternary complex at a resolution of 3.0 A of RT crosslinked to a dideoxy-terminated DNA with tenofovir-diphosphate bound as the incoming substrate; and (ii) a RT-DNA complex at a resolution of 3.1 A with tenofovir at the 3' primer terminus. The tenofovir nucleotide in the tenofovir-terminated structure seems to adopt multiple conformations. Some nucleoside reverse transcriptase inhibitors, including 3TC and AZT, have elements ('handles') that project beyond the corresponding elements on normal dNTPs (the 'substrate envelope'). HIV-1 RT resistance mechanisms to AZT and 3TC take advantage of these handles; tenofovir's structure lacks handles that could protrude through the substrate envelope to cause resistance.
Structures of HIV-1 RT-DNA complexes before and after incorporation of the anti-AIDS drug tenofovir.,Tuske S, Sarafianos SG, Clark AD Jr, Ding J, Naeger LK, White KL, Miller MD, Gibbs CS, Boyer PL, Clark P, Wang G, Gaffney BL, Jones RA, Jerina DM, Hughes SH, Arnold E Nat Struct Mol Biol. 2004 May;11(5):469-74. Epub 2004 Apr 25. PMID:15107837[36]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
See Also
References
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Braaten D, Franke EK, Luban J. Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription. J Virol. 1996 Jun;70(6):3551-60. PMID:8648689
- ↑ Anderson EC, Lever AM. Human immunodeficiency virus type 1 Gag polyprotein modulates its own translation. J Virol. 2006 Nov;80(21):10478-86. PMID:17041220 doi:80/21/10478
- ↑ Hagan NA, Fabris D. Dissecting the protein-RNA and RNA-RNA interactions in the nucleocapsid-mediated dimerization and isomerization of HIV-1 stemloop 1. J Mol Biol. 2007 Jan 12;365(2):396-410. Epub 2006 Oct 3. PMID:17070549 doi:http://dx.doi.org/10.1016/j.jmb.2006.09.081
- ↑ Ao Z, Danappa Jayappa K, Wang B, Zheng Y, Kung S, Rassart E, Depping R, Kohler M, Cohen EA, Yao X. Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication. J Virol. 2010 Sep;84(17):8650-63. doi: 10.1128/JVI.00508-10. Epub 2010 Jun 16. PMID:20554775 doi:10.1128/JVI.00508-10
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Smith CM, Leon O, Smith JS, Roth MJ. Sequence requirements for removal of tRNA by an isolated human immunodeficiency virus type 1 RNase H domain. J Virol. 1998 Aug;72(8):6805-12. PMID:9658129
- ↑ Tuske S, Sarafianos SG, Clark AD Jr, Ding J, Naeger LK, White KL, Miller MD, Gibbs CS, Boyer PL, Clark P, Wang G, Gaffney BL, Jones RA, Jerina DM, Hughes SH, Arnold E. Structures of HIV-1 RT-DNA complexes before and after incorporation of the anti-AIDS drug tenofovir. Nat Struct Mol Biol. 2004 May;11(5):469-74. Epub 2004 Apr 25. PMID:15107837 doi:10.1038/nsmb760
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