Evidence for the involvement of M1 protein in the formation of cord-like structures of influenza C virus

Abstract

We have succeeded in generating virus-like particles (VLPs) of influenza C virus from cloned cDNAs. A cDNA of green fluorescent protein (GFP) gene flanked by the 5' and 3' noncoding regions of the segment 5 of influenza C virus was inserted between an RNA polymerase I promoter and terminator of the Pol I vector in antisense orientation. This plasmid DNA was transfected into 293T cells together with plasmids encoding the viral proteins of C/Ann Arbor/1/50 (AA/50) or C/Yamagata/1/88 (YA/88). Transfer of the supernatants of the 293T cells to HMV-II cells with helper virus resulted in the GFP expression in the HMV-II cells, and the quantification of the GFP-positive HMV-II cells indicated the presence of 10 6 VLPs/ml supernatant. Cords with 50−300 μm in length were observed on the transfected 293T cells, though they were not observed when the plasmid for M1 protein of AA/50 was replaced with that of C/Taylor/1233/47 (TAY/47). This finding provided direct evidence for the previous hypothesis by Nishimura et al. (1994) that M1 protein is involved in the formation of cord-like structures protruded from the YA/88-infected cells.