A persistent variant of influenza C virus fails to interact with actin filaments during viral assembly

C/AA-pi virus, a variant of influenza C/Ann Arbor/1/50 virus, establishes persistent infections in MDCK cells, characterized by low levels of progeny production. During viral assembly, nucleoprotein (NP) was found homogeneously distributed over cytoplasmic and nuclear compartments and matrix (M) protein was likewise localized in a barely structured fashion. In contrast, infections with nonpersistent influenza A, B and C viruses produced cytoplasmic granular structures, which typically consisted of colocalized NP and M proteins. Studies on the in vitro interaction between NP and M proteins revealed identical binding capacities comparing influenza C wild-type virus with the persistent variant. Cytochalasin D treatment of infected cells demonstrated that NP protein of the wild-type virus, but not of the persistent variant, was distinctly associated with cellular actin filaments. Moreover, the assembly characteristics of wild-type virus were modulated in the presence of recombinant persistent-type NP protein towards a behaviour similar to persistent infection. Cell type specificity was particularly illustrated in C/AA-pi virus-infected Vero cells, which did not support viral persistence, but produced granular wild-type-like complexes. Thus, interaction between NP, M and actin proteins (i) is a basic part of the viral assembly process, (ii) is dominantly modulated by NP protein and (iii) is specifically altered in the case of persistent infection.