A previously unidentified host protein protects retroviral DNA from autointegration

Integration of a DNA copy of the viral genome into a host chromosome is an essential step in the retrovirus life cycle. The machinery that carries out the integration reaction is a nucleoprotein complex derived from the core of the infecting virion. To successfully integrate into host DNA, the viral DNA within this complex must avoid self-destructive integration into itself, a reaction termed autointegration. We have previously shown [Lee, M. S. and Craigie, R. (1994) Proc. Natl. Acad. Sci. USA 91, 9823-9827] that viral nucleoprotein complexes isolated from Moloney murine leukemia virus-infected cells exhibit a barrier to autointegration. This autointegration barrier could be destroyed by stripping factors from the complexes and subsequently restored by incubation with a host cell extract, but not by incubation with an extract of disrupted virions. We have now used this autointegration barrier reconstitution assay to purify the host factor from uninfected NIH 3T3 fibroblasts. It is a single polypeptide of 89 aa that does not match any previously identified protein. The identity of the protein was confirmed by expressing it in Escherichia coli and demonstrating the activity of the heterologously expressed protein in the reconstitution assay.