Construction and characterization of an SV40 mutant defective in nuclear transport of T antigen

An SV40-adenovirus 7 hybrid virus, PARA(cT), has been described that is defective for the nuclear transport of SV40 large tumor antigen. An SV40(cT) mutant was constructed using SV40 early and late region DNA fragments derived from PARA(cT) and wild-type SV40 respectively. The SV40(cT)-3 construct is defective for viral replication, but can be propagated in COS-1 cells. T antigen induced by SV40(cT)-3 is localized in the cytoplasm of infected cells. The cT mutation also inhibits the transport of wild-type T antigen; COS-1 cells lose their constitutive expression of nuclear T antigen after infection with SV40(cT)-3. Sequence analysis revealed that the cT mutation results in the replacement of a positively charged lysine in wild-type T antigen with a neutral asparagine at amino acid number 128, demonstrating that the alteration of a single amino acid is sufficient to abolish nuclear transport. Implications of the cT mutation on possible mechanisms for the transport of proteins to the nucleus are discussed.