Characterization of the nuclear localization signal in the avian sarcoma virus integrase

A sequence of 21 amino acids (aa) in the C-terminal region of the 286-aa avian sarcoma virus (ASV) integrase (IN) protein has been shown previously to mediate nuclear localization of both IN and beta-galactosidase (betaGal) protein fused to it. This karyophilic sequence includes a high proportion of prolines and residues with basic side chains. In this report, site-directed mutagenesis was used to introduce single aa substitutions of several of these residues. Indirect immunofluorescence showed that IN-betaGal fusion constructs with Ala substitutions for sequence constituents K206, P215, K225 or R227 had lost the exclusive nuclear localization capability of the wild-type fusion. A fusion protein with the conservative substitution K206R retained the nuclear localization capacity. The site-specific substitutions that reduced karyophilic activity had no effect on the processing or joining activities of IN in vitro. However, the introduction of three of the four Ala codon substitutions into viral DNA clones caused a significant delay in viral replication following transfection of cycling chicken embryo fibroblasts. These results are consistent with a possible role for ASV IN in nuclear targeting.