Asymmetric structure of the influenza A virus and novel function of the matrix protein M1

Influenza virus is an enveloped virus. It comprises two major modules: external lipoprotein envelope and internal ribonucleoprotein (RNP) containing the genomic negative-strand RNA. Lipoprotein envelope contains four vital proteins: hemagglutinin (HA), neuraminidase (NA), transmembrane ionic channel M2, and minor amounts of nuclear export protein NEP. RNP contains RNA and four polypeptides: major nucleocapsid protein NP and three polymerase subunits PB1, PB2, PA. Both modules are linked with each other by matrix M1 maintaining the virus integrity. According to the structural function, NP and M1 are predominant in virus particle in the amounts of 1000 and 3000 molecules, respectively. In addition to the structural function, M1 plays a role in regulation of intracellular and nuclear migration of viral RNP and virus assembly, referred as budding process, at the plasma membrane in infected cells. The bipolar structure of the influenza virus characterized by asymmetric location of RNP and nonregular distribution of M1 and M2 inside the virion is reviewed. The role of M1 in maintaining the asymmetric structure of the virus particle and regulation of RNP transport inside virus particle is considered. First experimental data confirming (i) intravirion RNP transport and its outside exit directed by the M1 and (ii) the importance of this process in virus uncoating and initiation of infection in target cell are discussed. A novel class of antiviral agents activating ATP-ase of the early endosome compartment in the target cell is discussed.