Viral Membrane Channels: Role and Function in the Virus Life Cycle

Sze, Ching Wooen; Tan, Yee-Joo

Another animal virus that manipulates the host calciomic to its own replication benefit is the diarrhea-inducing rotavirus. The hallmark of rotavirus infection is drastic changes in the calcium concentration, where ER calcium is leaked into the cytosol, activating the ER calcium sensor stromal interaction molecule 1 (STIM1), which subsequently leads to an influx of calcium through the plasma membrane and ultimately an increased in the cytoplasmic calcium level [109] . When the calcium store from ER is emptied into the cytoplasmic space, a protective calcium signaling pathway, calcium/calmodulin-dependent kinase kinase-β (CAMKK-β), is activated to initiate autophagy, a host mechanism for maintaining cellular homeostasis within the organism [110] . Rotavirus then hijacks this autophagy machinery and subverts it into a tool to transport its viral proteins to the replication site for assembly [42] . The viral protein responsible for the fluctuation of calcium store is none other than the NSP4 viroporin [49, [111] [112] [113] [114] . NSP4 is synthesized as an ER trans-membrane glycoprotein but can also exist in secreted form as enterotoxin to induce diarrhea by simultaneously activating secretion through a Ca 2+ -activated Cl − channel and inhibiting absorption by the epithelial Na + channel and the Na + /glucose co-transporter [115] . The domain responsible for the increase of cytosolic Ca 2+ is the viroporin domain from residue 47 to 90 as mutation of this region completely abolished its effect on calcium elevation [49] . NSP4 interacts with various host factors to facilitate different stages of the viral life cycle as well as contributing to the viral pathogenesis [116] [117] [118] [119] [120] . The enterotoxin form of NSP4 binds to integrins and activates calcium mobilization [120] , whereas the cytoplasmic tail of NSP4 can directly interact with the αand β-tubulin, acting as a viral microtubule-associated protein (MAP) [117] . Since NSP4 is synthesized in the ER, the ER-Golgi intermediate compartments of the infected cells will be coated with NSP4, leading to direct attachment of these vesicular compartments to the cytoskeleton and stalls further translocation [121] . Two studies showed that silencing of NSP4 led to a more diffuse distribution of viral proteins in the cytoplasm as well as a decrease in viral yield. Furthermore, Silverstri et al. went on to show that NSP4 acts as the modulator of viral transcription, where suppression of NSP4 expression prevented viroplasm maturation, the site of viral genome replication and packaging, as well as excessive transcription of viral RNA during the late infection cycle [122, 123] . All this evidence supports the essential role of NSP4 in the rotavirus lifecycle as a multifaceted viral protein that connects the different stages of viral replication events to ensure a productive infection.