Rotavirus protein expression is important for virus assembly and pathogenesis

Summary

Rotaviruses have a unique morphogenesis in which particles obtain a transient membrane-envelope as newly made subviral particles bud into the endoplasmic reticulum (ER). This process is mediated by a viral nonstructural glycoprotein, NSP4. We have found that NSP4 has pleiotropic properties that became evident following expression of this protein in eukaryotic cells. NSP4 expressed in insect cells bound double-layered rotavirus particles in a manner similar to receptor-ligand interactions and this interaction is thought to trigger the particle budding process. Expression of NSP4 in insect cells also increases intracellular calcium ([Ca²⁺]i) levels and this effect may explain the toxicity of this protein in eukaryotic cells. Increases in [Ca²⁺]i levels in insect cells also are observed following exogenous addition to cells of purified NSP4 or of a synthetic peptide of NSP4. Experiments to determine the mechanism by which NSP4 causes an increase in [Ca²⁺]i showed that Ca²⁺ is released from a subset of the thapsigargin-sensitive store [endoplasmic reticulum (ER)]. However, exogenously added and endogenously expressed NSP4 use different mechanisms to alter the Ca²⁺ permeability of the ER membrane. We hypothesize that NSP4-mediated changes in ER membrane permeability trigger viral budding into the lumen of the ER, and eventually induce cell death and release of virus particles from infected cells. We also propose that release of NSP4 following cell lysis and the concomitant stimulation of a Ca²⁺ signal transduction pathway in neighboring cells contributes to altered ion transport in intestinal epithelium resulting in diarrheal disease.