Nuclear localization of the adenovirus DNA-binding protein: requirement for two signals and complementation during viral infection.

The adenovirus DNA-binding protein (DBP) is an abundant multifunctional protein located primarily in the nuclei of infected cells. To define sequences involved in nuclear transport of DBP, a series of point and small deletion mutants were constructed via oligonucleotide-directed mutagenesis. Two short stretches of basic amino acids located in the amino-terminal domain (amino acids 42 to 46 and 84 to 89) were identified. Their importance, however, depended on the context in which DBP was expressed. Disruption of either site prevented nuclear localization after transient expression in transfected 293 cells, implying that two nuclear localization signals are necessary for transport of this nuclear protein. In contrast, the mutant DBPs synthesized during viral infection were located either primarily in the nucleus or in the nucleus and cytoplasm, depending on the mutation and the stage of the viral infection. Thus, the nuclear localization defect could be complemented by viral infection, perhaps through the interaction of the mutant polypeptide with a virus-encoded or -induced factor(s).