Although deletion of the entire C-terminal domain disrupted disintegration activity, an alanine substitution (P365A) in a conserved amino acid of the GP(Y/F) domain did not significantly reduce disintegration. When assayed for the ability to join two molecules of DNA in a reaction that modeled forward integration, the P365A substitution disrupted activity. UV cross-linking experiments detected DNA binding activity in the C-terminal domain and found that this activity was not reduced by substitutions in two conserved amino acids of the GP(Y/F) domain, G364A and P365A. Gel filtration and crosslinking of a 71-amino acid fragment containing the GP(Y/F) domain revealed a surprising ability to form dimers, trimers, and tetramers that was disrupted by the G364A and P365A substitutions. These results suggest that the GP(Y/F) residues may play roles in promoting multimerization and intermolecular strand joining.
Retroviruses and long terminal repeat (LTR)3 retrotransposons are closely related elements that depend on integrase (IN) to insert their cDNA into the genome of host cells. IN proteins are composed of three structurally distinct domains (1). The N-terminal domains contain an HHCC motif, and the catalytic core domains in the center of INs possess the DDE residues that mediate catalysis. The C-terminal domains of INs have little sequence conservation but possess nonspecific DNA binding activity.INs expressed as recombinant proteins possess a variety of catalytic activities. Oligonucleotides that model the termini of LTRs are trimmed by IN in a processing reaction that removes terminal nucleotides 3Ј of the conserved CA. Once the processing reaction is complete, strand transfer occurs. In this reaction, the 3Ј-hydroxyls of the terminal A serve as nucleophiles in transesterification reactions that cleave phosphodiester bonds in the target DNA and make covalent bonds between the 3Ј-ends of the viral DNA and the 5Ј-ends of the target DNA (2, 3). Under highly specific conditions, IN can catalyze concerted integration, the simultaneous insertion of two ends of donor DNA into the same site of target DNA (4 -7). In addition, INs also catalyze disintegration, the reverse of integration that uses model substrates that mimic one end of an LTR inserted into target DNA (8,9). This assay is a particularly sensitive method for measuring catalytic activity of INs, perhaps because it detects strand breaking and joining within a single substrate molecule, an intramolecular reaction.Currently, no molecular structures exist of an IN that possesses all three domains. The structures that do exist are of individual domains and do not include bound DNA. As a result, the function of the C-terminal domain in integration is not clear. An additional difficulty in determining the function of the C-terminal domains stems from their low levels of sequence conservation. However, close examination of C termini did identify two separate sequence modules that exist either alone or in combination in a wide variety of INs (10). One module termed the G...