Abstract
BackgroundThe 18 residue tail abutting the SH3 fold that comprises the heart of the C-terminal domain is the only part of HIV-1 integrase yet to be visualized by structural biology. To ascertain the role of the tail region in integrase function and HIV-1 replication, a set of deletion mutants that successively lacked three amino acids was constructed and analyzed in a variety of biochemical and virus infection assays. HIV-1/2 chimers, which harbored the analogous 23-mer HIV-2 tail in place of the HIV-1 sequence, were also studied. Because integrase mutations can affect steps in the replication cycle other than integration, defective mutant viruses were tested for integrase protein content and reverse transcription in addition to integration. The F185K core domain mutation, which increases integrase protein solubility, was furthermore analyzed in a subset of mutants.ResultsPurified proteins were assessed for in vitro levels of 3' processing and DNA strand transfer activities whereas HIV-1 infectivity was measured using luciferase reporter viruses. Deletions lacking up to 9 amino acids (1-285, 1-282, and 1-279) displayed near wild-type activities in vitro and during infection. Further deletion yielded two viruses, HIV-11-276 and HIV-11-273, that displayed approximately two and 5-fold infectivity defects, respectively, due to reduced integrase function. Deletion mutant HIV-11-270 and the HIV-1/2 chimera were non-infectious and displayed approximately 3 to 4-fold reverse transcription in addition to severe integration defects. Removal of four additional residues, which encompassed the C-terminal β strand of the SH3 fold, further compromised integrase incorporation into virions and reverse transcription.ConclusionHIV-11-270, HIV-11-266, and the HIV-1/2 chimera were typed as class II mutant viruses due to their pleiotropic replication defects. We speculate that residues 271-273 might play a role in mediating the known integrase-reverse transcriptase interaction, as their removal unveiled a reverse transcription defect. The F185K mutation reduced the in vitro activities of 1-279 and 1-276 integrases by about 25%. Mutant proteins 1-279/F185K and 1-276/F185K are therefore highlighted as potential structural biology candidates, whereas further deleted tail variants (1-273/F185K or 1-270/F185K) are less desirable due to marginal or undetectable levels of integrase function.
Highlights
The 18 residue tail abutting the SH3 fold that comprises the heart of the C-terminal domain is the only part of HIV-1 integrase yet to be visualized by structural biology
Retrovirus replication proceeds through a series of steps that initiate upon virus entry into a cell, followed by particle uncoating and reverse transcription
To ascertain the role of the tail in IN function, six nested deletions mutants lacking 3, 6, 9, 12, 15, or 18 amino acids from the C-terminus were constructed in the pKBIN6Hthr bacterial expression construct [39] and luciferase-based pNLX.Luc(R-) viral vector [42] (Figure 1)
Summary
The 18 residue tail abutting the SH3 fold that comprises the heart of the C-terminal domain is the only part of HIV-1 integrase yet to be visualized by structural biology. Because integrase mutations can affect steps in the replication cycle other than integration, defective mutant viruses were tested for integrase protein content and reverse transcription in addition to integration. Retrovirus replication proceeds through a series of steps that initiate upon virus entry into a cell, followed by particle uncoating and reverse transcription. The resulting double stranded cDNA must be integrated into a cell chromosome. DNA strand transfer, IN uses the 3'-oxygens to cut the chromosomal target DNA in a staggered fashion and at the same time joins the viral 3' ends to the resulting 5' phosphates [3]. The final step, repair of single stranded gaps and joining of viral DNA 5' ends, is accomplished by cellular enzymes [5,6]. HIV-1 IN activities can be measured in vitro using oligonucleotide DNA substrates that mimic the ends of the reverse transcript and either Mg2+ or Mn2+ cofactor [7,8,9,10]
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