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Novel PKC Is Required To Activate Replicative Functions of the Major Nonstructural Protein NS1 of Minute Virus of Mice

Applied Tumour Virology Program, Department F010 and Institut National de la Santé et de la Recherche Médicale U375, Deutsches Krebsforschungszentrum, Heidelberg, Germany

Received 10 February 2003/ Accepted 1 May 2003

The multifunctional protein NS1 of minute virus of mice (MVMp) is posttranslationally modified and at least in part regulated by phosphorylation. The atypical lambda isoform of protein kinase C (PKC) phosphorylates residues T435 and S473 in vitro and in vivo, leading directly to an activation of NS1 helicase function, but it is insufficient to activate NS1 for rolling circle replication. The present study identifies an additional cellular protein kinase phosphorylating and regulating NS1 activities. We show in vitro that the recombinant novel PKC phosphorylates NS1 and in consequence is able to activate the viral polypeptide in concert with PKC for rolling circle replication. Moreover, this role of PKC was confirmed in vivo. We thereby created stably transfected A9 mouse fibroblasts, a typical MVMp-permissive host cell line with Flag-tagged constitutively active or inactive PKC mutants, in order to alter the activity of the NS1 regulating kinase. Indeed, tryptic phosphopeptide analyses of metabolically ³²P-labeled NS1 expressed in the presence of a dominant-negative mutant, PKCDN, showed a lack of distinct NS1 phosphorylation events. This correlates with impaired synthesis of viral DNA replication intermediates, as detected by Southern blotting at the level of the whole cell population and by BrdU incorporation at the single-cell level. Remarkably, MVM infection triggers an accumulation of endogenous PKC in the nuclear periphery, suggesting that besides being a target for PKC, parvovirus infections may also affect the regulation of this NS1 regulating kinase. Altogether, our results underline the tight interconnection between PKC-mediated signaling and the parvoviral life cycle.

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