Biochemical features of protein matrix M1 of the influenza C virus

: Influenza viruses A, B, and C belonging to Orthomyxoviridae 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 treatment in a pH-independent manner. In contrast, disassembly of the M1 protein matrix appears to depend on pH. Treatment of influenza C viruses with nonionic detergent in neutral or alkaline medium (pH 9.0-7.2) 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 fails to be removed and the viral core-like complex of RNP along with the M1 matrix cover is released. Since influenza A and B viruses were characterised by acid-dependent disassembly of the virion M1 matrix, influenza C viruses seem to be more resemble the paramyxoviruses, which also show a neutral-alkaline pH dependence of the matrix disintegration. These observations suggest that uncoating of influenza C viruses and paramyxoviruses in target cells may have similar events.