Influenza A M2 recruits M1 to the plasma membrane: a fluorescence fluctuation microscopy study

Abstract

Influenza A virus (IAV) is a respiratory pathogen that causes seasonal epidemics and occasional pandemics of severe illnesses with significant mortality. One of the most abundant proteins in IAV particles is the matrix protein 1 (M1), which is essential for the structural stability of the virus. M1 organizes virion assembly and budding at the plasma membrane (PM), where it can interact with other viral components and cellular membrane factors (i.e. lipids and host proteins). Of interest, the recruitment of M1 to the PM as well as its interaction with the other viral envelope proteins (hemagglutinin (HA), neuraminidase, matrix protein 2 (M2)) is controversially discussed in previous studies. Therefore, we used fluorescence fluctuation microscopy techniques (i.e. (cross-correlation) number and brightness, and scanning fluorescence cross-correlation spectroscopy) to quantify the oligomeric state of M1 and its interaction with other viral proteins in co-transfected as well as infected cells. Our results indicate that M1 is recruited to the PM by M2, as a consequence of the strong interaction between the two proteins. In contrast, only a weak interaction between M1 and HA was observed. M1-HA interaction occurred only in the case that M1 was already bound to the PM. We therefore conclude that M2 initiates the assembly of IAV by recruiting M1 to the PM, possibly allowing its further interaction with other viral proteins.