The Effect of Protease Treatment on the Morphology of Influenza A, B and C Viruses

Since the partial degradation of influenza virus particles by trypsin was first reported (Mayron & Rafelson, 1960), the virus has been shown to be susceptible to the action of many other proteases. Degradation by these means is known to cause the release of low molecular weight neuraminidase from the particles, but the other activities associated with the viral surface, that is haemagglutination and V-antigen complement fixation, are apparently destroyed. Degraded virus particles obtained after such treatment show markedly altered physical properties which may be attributed to the loss of about 50% of the viral protein; the bulk of nucleic acid however is still particle-associated (Biddle, 1968; Kendal, Biddle & Belyavin, 1968). This suggests that there is a membrane resistant to protein digestion beneath the characteristic protein outer coat of the virus. To investigate this structural feature, protease digested virus was examined electron microscopically, using both negative staining and thin-sectioning techniques.