- Endocytosis or veropexis is the most common cellular entry technique for viruses
- Once the cell internalizes the virus, it is then delivered to an acidic pit, a so-called early endosome. The virus then may be transferred into a late endosome and then to a lysosome.
- Alternatively, due to the low pH value in the lumen of endosomes, the viral membrane can fuse with the endosomal membrane, releasing the viral genome into the cytoplasm.
- After exiting from endosomes, some adenoviruses or poxviruses may use microtubules for transport within the cytoplasm.
- Once in the cytoplasm, some viruses move toward the nucleus to deliver their cargo inside the nucleus, whereas the nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae usually remain in the cytoplasm to initiate their replication cycle.
- Dynamin GTPase may have a key role in regulating most endocytic pathways.
- During virus entry, dynamin is deposited in the neck of the endocytic pit toward the cytoplasm leading to the excision of the pit.
- There are several major endocytosis-based pathways that viruses can use to enter cells and evade the host’s immune system.
- These pathways differ in terms of the types of particles involved and the molecules that are important in the process. The most important viral entry pathways are as follows:
a) Pinocytosis (cell drinking), which is the process by which cells take up solutes and fluids. Pinocytotic processes can be further classified based on the membrane structures and types of molecules they are associated with. Macropinocytosis is a nonspecific process, and particles internalized by this route may not be essential for the cell. When it is exploited by viruses, interactions between viral proteins and cell receptors activate intracellular signaling and actin rearrangements that form ruffles or filopodia on the external surface of the host cell. The ruffles then close up to form a vesicle known as a macropinosome, which carries the virus into the cytosol. Actin, Rho GTPases (Rac and Cdc42), PI3K, and Na+/H+ exchange are usually required for this pathway, and kinases are required to regulate macropinosome formation and closure.