Disassembly of influenza C viruses, distinct from that of influenza A and B viruses requires neutral-alkaline pH.

Influenza C (Flu C) viruses comprise an internal ribonucleoprotein (RNP) and an outer lipoprotein envelope with surface spike glycoproteins and the M1 protein matrix. The lipoprotein envelope and spike glycoproteins are solubilized by nonionic detergent in a pH-independent manner. In contrast, disassembly of the M1 protein matrix appears to depend on pH. Treatment of Flu C viruses with nonionic detergent in neutral or alkaline medium (pH 9.0-7.4) results in disintegration of the virion M1 matrix and leads to a significant release of RNP free of the M1 protein. In acidic medium (pH 6.0-5.0), the M1 matrix is not removed and the viral core-like complex of RNP along with the M1 matrix cover is released. Since Flu A and B viruses were characterized by acid-dependent disassembly of the virion M1 matrix, Flu C viruses seem to resemble the paramyxoviruses, which also show a neutral-alkaline pH dependence on matrix disintegration. These observations suggest that uncoating mechanisms of influenza C viruses and paramyxoviruses in target cells may be similar.