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. 2021 Nov 23;14(12):1210. doi: 10.3390/ph14121210

Figure 3.

Figure 3

Structure and replication cycle of the Influenza A virus. According to Neumann et al. [57] and Shi et al. [58], Influenza A virus consists of structural proteins present in the lipid bilayer envelope (LBE), including hemagglutinin (HA), neuraminidase (NA), the matrix protein M1, and ion channel protein M2. It has a negative-sense single-stranded RNA (−ssRNA) genome that contains eight gene segments that encode 16 proteins (although not all influenza viruses express all 16 proteins). Its genomic RNA is encapsulated by nucleoprotein (NP) and components of the RNA-dependent RNA polymerase complex (PB1, PB2, and PA). The replication cycle of the influenza A virus is initiated by the binding of the virus HA to the sialylated receptors (α2,3 or α2,6-linked sialic acid) on the surface of the host cell. This allows endocytosis-mediated entry of the virus. Following the fusion of the virus and host cell membranes, uncoating occurs and the release of viral RNA into the cytoplasm. Subsequently, the −ssRNA genome (noncoding) is transported to the nucleus, where replication and transcription into coding RNA (+ssRNA) occur. Messenger RNAs are exported to the cytoplasm for translation. The early viral proteins, that is, those necessary for replication and transcription (NP, NS1, PA, PB1, PB2, PB1-F2), are transported back to the nucleus. After synthesis in the cytoplasm, NP proteins stabilize the −ssRNA in the nucleus. Genomic RNA with RNA polymerase, NP, matrix proteins, and packaging proteins are exported from the nucleus to the cytoplasm with the help of M1 and NS2 proteins (late viral proteins). The envelope proteins produced in the endoplasmic reticulum (ER) move through the transport vesicle from the ER to the Golgi apparatus and then to the plasma membrane. Finally, the genomic RNA and the viral protein complex are packaged into progeny viruses as they emerge from the cell membrane by exocytosis. The red box indicates the potential mechanism of action against influenza A virus by 1,8-cineole and other monoterpenes present in Eucalyptus essential oil by binding and inhibiting the hemagglutinin protein and thus inhibiting the binding of the virus with its receptor and subsequent entry into the host cell.