FUNCTIONAL COMPLEMENTATION OF NUCLEAR TARGETING-DEFECTIVE MUTANTS OF SIMIAN-VIRUS-40 STRUCTURAL PROTEINS

Structural proteins of simian virus 40 (SV40), Vp2 and Vp3 (Vp2/3) and Vp1, carry individual nuclear targeting signals, Vp3(198-206) (Vp2(31 6-324)) and Vp1(1-8), respectively, which are encoded in different rea ding frames of an overlapping region of the genome. How signals coordi nate nuclear targeting during virion morphogenesis was examined by usi ng SV40 variants in which there is only one structural gene for Vp1 or Vp2/3, nuclear targeting-defective mutants thereof, Vp2/3(202T) and V p1 Delta N5, or nonoverlapping SV40 variants in which the genes for Vp 1 and Vp2/3 are separated, and mutant derivatives of the gene carrying either one or both mutations. Nuclear targeting was assessed immunocy tochemically following nuclear microinjection of the variant DNAs. Whe n Vp2/3 and Vp1 mutants with defects in the nuclear targeting signals were expressed individually, the mutant proteins localized mostly to t he cytoplasm. However, when mutant Vp2/3(202T) was coexpressed in the same cell along with wild-type Vp1, the mutant protein was effectively targeted to the nucleus. Likewise, the Vp1 Delta N5 mutant protein wa s transported into the nucleus when wild-type Vp2/3 was expressed in t he same cells. These results suggest that while Vp1 and Vp2/3 have ind ependent nuclear targeting signals, additional signals, such as those defining protein-protein interactions, play a concerted role in nuclea r localization along with the nuclear targeting signals of the individ ual proteins.