As NSs lacking C-terminus exhibited slight dominant-negative effect on PKR degradation, we hypothesized that an intact sequence at aa.1 to 248 is required for the dominant-negative effect. We next tested the effect of the C-terminus on the dominant-negative effect. We substituted two sequential acidic amino acids triplets located at the C-terminus with alanines as shown in Fig. 7A; i.e., NSs-E253-255A/D257-259A, NSsD257-259A or NSs-E253-255A. We found that NSs-E253-255A could form filamentous inclusion bodies (data not shown), and accumulation was equivalent to that of MP-12 NSs (Fig. 7B) . On the other hand, NSs-E253-255A/D257-259A or NSsD257-259A did not accumulate in cells efficiently, and the NSs could not be detected with IFA (data not shown). We then tested PKR degradation function and IFN-b mRNA suppression function of those mutants using VeroE6 cells and A549 cells using the same method as described above. Cells infected with NSs-E253-255A degraded PKR (Fig. 7B) , and inhibited the synthesis of IFN-b mRNA (Fig. 7C) , while those infected with NSs-E253-255A/ D257-259A or NSsD257-259A did not degrade PKR and did not inhibit IFN-b mRNA synthesis. The results suggest that the glutamic acid at aa.253 to 255 can be replaced without affecting [26] and C-terminus selfassociation domain (aa.249-265) [65] . The rMP12-C13type (C13type) encodes an in-frame truncation of aa. 16-198, NSs functions, while aspartic acid at aa.257 to 259 is important for NSs stability.