section_path: Virology/Genome and structure
section_name: Genome and structure
paragraph_text: Genome and structure
  
  Influenza viruses have a negative-sense, single-stranded RNA genome that is segmented. The negative sense of the genome means it can be used as a template to synthesize messenger RNA (mRNA). Influenza A virus and influenza B virus have eight genome segments that encode 10 major proteins. Influenza C virus and influenza D virus have seven genome segments that encode nine major proteins.
  
  Three segments encode three subunits of an RNA-dependent RNA polymerase (RdRp) complex: PB1, a transcriptase, PB2, which recognizes 5' caps, and PA (P3 for influenza C virus and influenza D virus), an endonuclease. The M1 matrix protein and M2 proton channel share a segment, as do the non-structural protein (NS1) and the nuclear export protein (NEP). For influenza A virus and influenza B virus, hemagglutinin (HA) and neuraminidase (NA) are encoded on one segment each, whereas influenza C virus and influenza D virus encode a hemagglutinin-esterase fusion (HEF) protein on one segment that merges the functions of HA and NA. The final genome segment encodes the viral nucleoprotein (NP). Influenza viruses also encode various accessory proteins, such as PB1-F2 and PA-X, that are expressed through alternative open reading frames and which are important in host defense suppression, virulence, and pathogenicity.
  
  The virus particle, called a virion, is pleomorphic and varies between being filamentous, bacilliform, or spherical in shape. Clinical isolates tend to be pleomorphic, whereas strains adapted to laboratory growth typically produce spherical virions. Filamentous virions are about 250 nanometers (nm) by 80 nm, bacilliform 120–250 by 95 nm, and spherical 120 nm in diameter.
  
  The core of the virion comprises one copy of each segment of the genome bound to NP nucleoproteins in separate ribonucleoprotein (RNP) complexes for each segment. There is a copy of the RdRp, all subunits included, bound to each RNP. The genetic material is encapsulated by a layer of M1 matrix protein which provides structural reinforcement to the outer layer, the viral envelope. The envelope comprises a lipid bilayer membrane incorporating HA and NA (or HEF) proteins extending outward from its exterior surface. HA and HEF proteins have a distinct "head" and "stalk" structure. M2 proteins form proton channels through the viral envelope that are required for viral entry and exit. Influenza B viruses contain a surface protein named NB that is anchored in the envelope, but its function is unknown.