Solution structure of the N-terminal zinc binding domain of HIV-1 integrase

Critical contacts between HIV-1 integrase and viral DNA identified by structure-based analysis and photo-crosslinking

Analysis of the crystal structure of HIV-1 integrase reveals a cluster of lysine residues near the active site. Using site-directed mutagenesis and photo-crosslinking we find that Lys156 and Lys159 are critical for the functional interaction of integrase with viral DNA. Mutation of Lys156 or Lys159 to glutamate led to a loss of both 3' processing and strand transfer activities in vitro while maintaining the ability to interact with nonspecific DNA and support disintegration. However, mutation of both residues to glutamate produced a synergistic effect eliminating nearly all nonspecific DNA interaction and disintegration activity. In addition, virus containing either of these changes was replication-defective at the step of integration. Photo-crosslinking, using 5-iododeoxyuracil-substituted oligonucleotides, suggests that Lys159 interacts at the N7 position of the conserved deoxyadenosine adjacent to the scissile phosphodiester bond of viral DNA. Sequence conservation throughout retroviral integrases and certain bacterial transposases (e.g. Tn10/IS10) supports the premise that within those families of polynucleotidyl transferases, these residues are strategic for DNA interaction.

Structure, biological functions and inhibition of the HIV-1 proteins Vpr and NCp7

The Gag-encoded nucleocapsid protein NCp7 (72 amino acids) from HIV-1, the regulatory protein, Vpr (96 amino acids) and numerous derivatives have been synthesized by solid phase method and their structures determined by 2D NMR. In NCp7, the two highly folded zinc fingers of the Cx2Cx4Hx4C type are in close spacial proximity and the replacement of H by C in the first zinc finger or P by L in the short interdigital domain led to structural modifications evidenced by NMR. In vivo, these point mutations induced a complete loss of viral infectivity by interrupting critical step(s) of the retroviral life cycle. To account for these findings, a model of the complex between NCp7 and d (ACGCC) has been proposed from NMR data, showing the intercalation of Trp37 in the oligonucleotide. This model could also explain the role of NCp7 in the formation of viral particles and agrees with the modifications in morphology of the virions containing mutations in the NCp7 zinc fingers. Vpr is essentially constituted by two long helical domains at its N- and C-terminals and the side chains of Leu60 and Leu67 participate in a leucine-zipper mode of intramolecular interaction. The results obtained have been used to try to develop new antiviral agents inhibiting NCp7 functions and thus possibly devoid of the resistance effects found with inhibitors of HIV enzymes (reverse transcriptase and protease).

The solution structure of the amino-terminal HHCC domain of HIV-2 integrase: a three-helix bundle stabilized by zinc

BACKGROUND

Integrase mediates a crucial step in the life cycle of the human immunodeficiency virus (HIV). The enzyme cleaves the viral DNA ends in a sequence-dependent manner and couples the newly generated hydroxyl groups to phosphates in the target DNA. Three domains have been identified in HIV integrase: an amino-terminal domain, a central catalytic core and a carboxy-terminal DNA-binding domain. The amino-terminal region is the only domain with unknown structure thus far. This domain, which is known to bind zinc, contains a HHCC motif that is conserved in retroviral integrases. Although the exact function of this domain is unknown, it is required for cleavage and integration.

RESULTS

The three-dimensional structure of the amino-terminal domain of HIV-2 integrase has been determined using two-dimensional and three-dimensional nuclear magnetic resonance data. We obtained 20 final structures, calculated using 693 nuclear Overhauser effects, which display a backbone root-mean square deviation versus the average of 0.25 A for the well defined region. The structure consists of three alpha helices and a helical turn. The zinc is coordinated with His 12 via the N epsilon 2 atom, with His16 via the N delta 1 atom and with the sulfur atoms of Cys40 and Cys43. The alpha helices form a three-helix bundle that is stabilized by this zinc-binding unit. The helical arrangement is similar to that found in the DNA-binding domains of the trp repressor, the prd paired domain and Tc3A transposase.

CONCLUSION

The amino-terminal domain of HIV-2 integrase has a remarkable hybrid structure combining features of a three-helix bundle fold with a zinc-binding HHCC motif. This structure shows no similarity with any of the known zinc-finger structures. The strictly conserved residues of the HHCC motif of retroviral integrases are involved in metal coordination, whereas many other well conserved hydrophobic residues are part of the protein core.

Crystal structure of the specific DNA-binding domain of Tc3 transposase of C.elegans in complex with transposon DNA

The crystal structure of the complex between the N-terminal DNA-binding domain of Tc3 transposase and an oligomer of transposon DNA has been determined. The specific DNA-binding domain contains three alpha-helices, of which two form a helix-turn-helix (HTH) motif. The recognition of transposon DNA by the transposase is mediated through base-specific contacts and complementarity between protein and sequence-dependent deformations of the DNA. The HTH motif makes four base-specific contacts with the major groove, and the N-terminus makes three base-specific contacts with the minor groove. The DNA oligomer adopts a non-linear B-DNA conformation, made possible by a stretch of seven G:C base pairs at one end and a TATA sequence towards the other end. Extensive contacts (seven salt bridges and 16 hydrogen bonds) of the protein with the DNA backbone allow the protein to probe and recognize the sequence-dependent DNA deformation. The DNA-binding domain forms a dimer in the crystals. Each monomer binds a separate transposon end, implying that the dimer plays a role in synapsis, necessary for the simultaneous cleavage of both transposon termini.