NS-1 and NS-2 proteins may act synergistically in the cytopathogenicity of parvovirus MVMp

The interaction of parvovirus minute virus of mice (prototype strain, MVMp) with simian virus 40 (SV40)-transformed human cells (NB-E) was investigated by means of transfection with MVMp molecular clones derived from the infectious recombinant plasmid (pMM984). pMM984 inhibits stable transformation of NB-E cells to geneticin resistance (G418 R) upon cotransfection with the selectable pSV2neo plasmid. We show here that this inhibition is not merely caused by a repression of marker gene expression from the SV40 early region promoter in pSV2neo and rather is likely to reflect the cytotoxic action of the parvovirus. Starting from plasmid pMM984, defined mutations were introduced into the genome of MVMp and more particularly into sequences coding for the NS-1 and/or NS-2 nonstructural proteins. In this way we could show that the NS-1 protein is necessary for the inhibition of transformation to G418 R and that the NS-2 protein acts synergistically to enhance this effect. Moreover, results obtained with different viral mutants indicate that the inhibitory action of NS-1 on stable transformation can be dissociated from the ability of this protein both to transactivate the parvoviral p39 promoter of the capsid protein-encoding region and to drive parvoviral DNA amplification. Altogether these data point to a probable direct toxicity of MVMp nonstructural proteins for permissive host cells.