Required for the expression of the capsid proteins. Performs the splicing and internal polyadenylation of the viral capsid-encoding mRNA precursor, which allows its maturation and expression (PubMed:26912614). Transactivates the viral promoter (PubMed:24161033). Abolishes viral capsid-encoding mRNAs in the host cytoplasm with concomitant loss of expression of the capsid proteins. Belongs to the Bocaparvovirus Non-structural protein NP-1 family.