Kinetic analysis of avian sarcoma virus integrase in the presteady-state

Graduation date: 2003Integrase catalyzes insertion of a retroviral genome into the host chromosome.\ud Following reverse transcription, integrase binds specifically to the ends of the duplex retroviral\ud DNA, endonucleolytically cleaves two nucleotides from each 3'-end (the processing activity),\ud and inserts these ends into the host DNA (the joining activity) in a concerted manner.\ud Additionally, it has been observed that integrase can catalyze the removal of inserted viral ends\ud (the disintegration activity) in vitro. Presteady-state experiments were performed using synapsed\ud substrates to probe the processing reaction and a disintegration substrate to determine the\ud number of protomers in a functional multimeric complex. In single-turnover studies, a novel\ud "splicing" reaction was observed that revealed complications with accurate quantification of\ud enzymatic activity using the synapsed substrates. The splicing reaction was further used to gain\ud insight into the selection of nucleophiles and electrophiles at the binding site. To reduce the\ud complexity introduced by the integrase-catalyzed splicing reaction, 5'-5' reverse-polarity\ud synapsed substrates were designed that were not susceptible to the splicing reaction and that\ud allowed direct comparison of LTR ends simultaneously bound at the active site. Analysis of the\ud presteady-state assays using these reverse-polarity substrates revealed that the concurrent binding\ud of the biologically relevant U3/U5 combination of viral ends facilitates maximal activity of the\ud processing reaction. A disintegration substrate was used in presteady-state active site titrations to\ud determine a reaction stoichiometry of four integrase protomers per one substrate molecule for\ud the disintegration reaction. A tetrameric active complex was then confirmed using atomic force\ud microscopy to image integrase-DNA complexes during the first catalytic turnover. The\ud observed increase of the tetramer population in the presence of substrate DNA demonstrates that\ud the binding of the disintegration substrate induces assembly of the active tetramer and suggests\ud that tetramer assembly may be an integral and dynamic component of the catalytic pathway