A hypothesis about the mechanism of assembly of double-shelled rotavirus particles.

During double-shelled (ds) particle assembly, subviral particles [possibly single-shelled (ss) particles] acquire the outer capsid protein during their transport across the endoplasmic reticulum (ER) membrane by an exocytosis-like process, probably by a fusion-like mechanism. Fine reticular material is observed around the junction area between virus particles and the ER membrane on the cytoplasmic side of projecting ss particles, suggesting this is the site of assembly of ds particles. It is assumed that the reticular material may correspond to the hetero-oligometric complexes consisting of the non-structural glycoprotein NSP4, the structural proteins VP4 and VP7, and that both VP7 and VP4 may fold onto ss particles as a complex. On the other hand, the budding process simply serves as a vehicle to transport ss particles from the cytoplasm to the ER lumen. Thus, it is assumed that the production of protein complexes may be indispensable for virion assembly, in which NSP4 regulates VP4 folding as an ER chaperone and also the exocytosis-like or fusion-like transport systems through the ER membrane.