Non-structural glycoprotein 4, NSP4 or enterotoxin, of 175 aas and 2 TMSs (Hyser et al. 2012).  A pentatmeric structure of a 53 aas NSP4 fragment has been solved (3MIW).  NSP4 viroporin is involved in activation.  It increases the  endoplasmic reticulum (ER) permeability, resulting in decreased ER calcium stores and activation of plasma membrane (PM) calcium influx channels, ultimately causing the elevation in cytoplasmic calcium (Hyser et al. 2013).  It activates ER calcium store-operated calcium entry (Hyser et al. 2013). NSP4 VPD is a Ca^2+/Ba²⁺-conducting cation-selective viroporin that transports monovalent and divalent cations equally well (Pham et al. 2017). It may be involved in particle production (Scott and Griffin 2015).