ИСТИНА

Influenza A viruses are important pathogens that still rank among the major global health problems. Matrix protein M1 in the virus particles is one of the most important and abundant proteins broadly involved in essential processes of the viral life cycle. It makes the structural investigation of the M1 protein particularly important. We employed synchrotron small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM) to study the structure of M1 at different conditions. The low-resolution structural models built from the SAXS data reveal a structurally anisotropic M1 molecule consisting of a compact NM-fragment and an extended and partially flexible C-terminal domain [1]. We demonstrated also that even at low pH the M1 monomers co-exist in solution with a small fraction of large clusters possessing a layered architecture similar to that observed in the authentic influenza virions. AFM analysis on a lipid-like negatively charged surface reveals that M1 forms ordered stripes correlating well with the clusters observed by SAXS [1]. Low pH condition occurs at the very beginning of cell infection leading to an acid-triggered fusion of the viral membrane. Moreover, change of pH was found to serve as a impuls allowing M1 to carry out its multiple functions in the uncoating, nuclear transport, and assembly of the viral ribonucleocapsid [2]. Revealed by us helix-like shapes could be treated as pre-matrix protein superstructures, whose formation is an intrinsic biological property of the M1 protein. It can be assumed, however, that the oligomerization of M1 should strongly depend on pH and on the protein charge. That is why it was important to analyse the structure and self-assembly of M1 at gradually changing pH (up to the neutral pH condition) in solution and on the bare mica surface using SAXS and AFM, correspondingly. We found that the oligomerization processes occur in a similar way in the solution and on the substrate, and quantitatively described these processes. Moreover, pH 6.0 was found to be the condition at which binding between M1 molecules starts to break. Our results provide new insights into the mechanism of M1 to form matrix and virus-like particles alone without partners and give a basis for a further analysis of the hierarchy of M1 in the virus life cycle. This work was supported in part by Russian Foundation for Basic Researches (projects 15-54-74002 EMBL and 16-04-00563) References 1. E. Shtykova et al. PLoS One, 8, (2013) e82431. 2. M. Bu et al. J. Virology, 70, (1996) 8391-8401.