In Situ Arrangement Of Influenza A Virus Matrix Protein M1 Resolved By Cryo Electron Tomography Suggests A Model For Virus Assembly

Influenza A virus (IAV) assembles into pleomorphic particles at the plasma membrane of infected cells. Genetically, matrix protein 1 (M1) has been identified as a key contributor to morphology determination and virus assembly. Previously, M1 has been observed to form a protein layer underneath the viral envelope of IAV particles by cryo electron microscopy (cryoEM). However, details of the arrangement of M1 within this protein layer have remained unknown. Here, we have applied cryo electron tomography (cryoET) in combination with subtomogram averaging to investigate the 3D-arrangement and structure of the M1 protein directly from non-purified IAV virions of the influenza A/Hong-Kong/1968 strain (HK68). By applying subtomogram averaging to individual virus filaments, we revealed that M1 monomers are arranged as helical strands underneath the viral envelope but with different helix start numbers in different viruses and without a fixed helical symmetry. We then compared the data on M1 arrangement to a theoretical model, which describes M1 polymerization and membrane deformation and found that tight packing of linearly polymerized helices of M1 underneath the membrane provides enough energy to overcome membrane resistance. Our data in combination with results from the theoretical model therefore suggest that M1 polymerization ultimately provides the driving force for IAV assembly.