THE ROTAVIRUS NONSTRUCTURAL GLYCOPROTEIN NSP4 MOBILIZES CA2+ FROM THE ENDOPLASMIC-RETICULUM

We previously reported that expression of rotavirus nonstructural glycoprotein NSP4 is responsible for an increase in cytosolic free Ca2+ Concentration ([Ca2+](i)) in Spodoptera frugiperda (Sf9) insect cells (P, Tian, Y, Hu, W, P, Schilling, D, A, Lindsay, J, Eiden, and M, K, Estes, J, Virol, 68:251-257, 1994) The purpose of the present study was to determine the mechanism by which NSP4 causes an increase in [Ca2+](i) by measuring the permeability of the cytoplasmic and endoplasmic reticulum (ER) membranes in recombinant-baculovirus-infected Sf9 cells, No obvious change in plasmalemma permeability to divalent cations was observed in cells expressing NSP4 compared with that in cells expressing another rotaviral glycoprotein (VP7) when the influx of Ba2+ a Ca2+ surrogate, was monitored, The basal Ca2+ permeability of the internal Ca2+ store was evaluated by measuring the release of Ca2+ induced by ionomycin, a Ca2+ ionophore, or thapsigargin, an inhibitor of the ER Ca2+-ATPase pump, following suspension of the cells in Ca2+-free extracellular buffer, Releasable Ca2+ decreased with time to a greater extent in cells expressing NSP4 compared with that in cells expressing VP7, suggesting that NSP4 increases the basal Ca2+ permeability of the ER membrane, To determine the possible mechanism by which NSP4 increases ER permeability, purified NSP4 protein or a 22-amino-acid synthetic peptide consisting of residues 114 to 135 (NSP4(114-135)) was added exogenously to noninfected Sf9 cells during measurement of [Ca2+](i). Both NSP4 and the NSP4(114-135) peptide produced a time-dependent increase in [Ca2+](i) that was attenuated by prior inhibition of phospholipase C with U-73122, Pretreatment of the cells with thapsigargin completely blocked the increase in [Ca2+], produced by NSP4(114-135), but the peptide only partially reduced the change in [Ca2+], produced by thapsigargin, No changes in [Ca (+)](i) were Seen in cells treated with control peptides, These results suggest that (i) exogenous NSP4 increases [CP2+](i) through the activation of phospholipase C, (ii) Ca2+ release by exogenous NSP4 is from a store that is a subset of the thapsigargin-sensitive compartment, and (iii) amino acid residues 114 to 135 of NSP4 are sufficient for this activity, In contrast to exogenous NSP4, the mechanism by which endogenously expressed NSP4 increases [Ca2+](i) appears to be unrelated to phospholipase C, since no effect of U-73122 was seen on the elevated [Ca2+](i) in cells expressing NSP4 and exogenously applied NSP4(114-135) caused a further increase in [Ca2+](i) in cells expressing NSP4 protein, We propose that alteration in the Ca2+ permeability of the ER membrane during viral expression of NSP4 plays an important role in virus maturation and that release of NSP4 following cell lysis and the concomitant stimulation of Ca2+ signal transduction in neighboring cells may contribute to altered ion transport in the intestinal epithelium. The actual mechanism by which NSP4 alters ER permeability is unknown, NSP4 may (i) form a cation channel in the ER membrane, (ii) increase Ca2+ leakage through the inositol-1,4,5-trisphosphate receptor, and/or (iii) have action on the phospholipids leading to membrane disruption.