Intracellular localization of Herpes simplex virus type 1 thymidine kinase fused to different fluorescent proteins depends on choice of fluorescent tag.

Gene therapy employing the suicide gene/prodrug activating system Herpes simplex virus type 1 thymidine kinase (HSV-TK)/ganciclovir (GCV) is effective in killing malignant tumor cells. Labeling of the HSV-TK enzyme with fluorescent proteins makes possible the non-invasive imaging of transduction efficiency, enzyme localization and activity in cell culture and in animal models of human cancers. Here we report the expression of HSV-TK tagged with different fluorescent proteins (EGFP, DSRed1, DsRed2, dsdrFP616) and show that intracellular localization of the fusion products depends on the nature of the fluorescent tag despite the presence of several nuclear targeting signals within the enzyme itself. Coexpression of red fluorescent HSV-TK fusion proteins with TK-EGFP or untagged HSV-TK allowed these proteins to enter the nucleus by inhibiting formation of red fluorescent protein oligomers. As enzyme localization may influence HSV-TK activity, this observation is of potential importance to gene therapy studies.