NP1 Protein of the Bocaparvovirus Minute Virus of Canines Controls Access to the Viral Capsid Genes via Its Role in RNA Processing

Minute virus of canines (MVC) is an autonomous parvovirus in the genus Bocaparvovirus. It has a single promoter that generates a single pre-mRNA processed via alternative splicing and alternative polyadenylation to produce at least 8 mRNA transcripts. MVC contains two polyadenylation sites, one at the right-hand end of the genome, (pA)d, and another complex site, (pA)p, within the capsid-coding region. During viral infection, the mRNAs must extend through (pA) p and undergo additional splicing of the immediately upstream 3D/3A intron to access the capsid gene. MVC NP1 is a 22-kDa nuclear phosphoprotein unique to the genus Bocaparvovirus of the Parvovirinae which we have shown governs suppression of (pA) p independently of viral genome replication. We show here that in addition to suppression of (pA) p, NP1 is also required for the excision of the MVC 3D/3A intron, independently of its effect on alternative polyadenylation. Mutations of the arginine/serine (SR) di-repeats within the intrinsically disordered amino terminus of NP1 are required for splicing of the capsid transcript but not suppression of polyadenylation at (pA) p. 3'-end processing of MVC mRNAs at (pA) p is critical for viral genome replication and the optimal expression of NP1 and NS1. Thus, a finely tuned balance between (pA) p suppression and usage is necessary for efficient virus replication. NP1 is the first parvovirus protein implicated in RNA processing. Its characterization reveals another way that parvoviruses govern access to their capsid protein genes, namely, at the RNA level, by regulating the essential splicing of an intron and the suppression of an internal polyadenylation site.