Subcellular localization of the MNV-1 ORF1 proteins and their potential roles in the formation of the MNV-1 replication complex

Human noroviruses are the leading cause of nonbacterial gastroenteritis worldwide and are now recognised as a significant human pathogen. Whereas human noroviruses cannot be cultivated in the laboratory, mouse norovirus 1 (MNV-1) is easily cultivated and has a defined tropism for cells of a mononuclear origin. As such, MNV-1 provides an ideal opportunity to study many aspects of norovirus biology and replication. Previously, we have shown that MNV-1 RNA replication is associated with components of the early and late secretory pathway and that all six open reading frame 1 (ORF1) proteins are associated with the viral dsRNA within the replication complex (RC) during the course of infection. In this study, we further characterise the subcellular localisation of the MNV-1 ORF1 proteins when recombinantly expressed in cells. We show that two MNV-1 proteins, NS1-2 and NS4, associate with the endoplasmic reticulum and endosomes, respectively. Whereas NS6 (the viral protease) appeared to localize within the cytoplasm and to mitochondria, NS7 (the viral polymerase) was observed to localize diffusely within the cytoplasm and within the nucleus, and NS3 localized to discrete foci within the cytoplasm which were of unknown origin. Based on the localization patterns observed we propose a model by which NS1-2 and NS4 may recruit host membranes to the MNV-1 RC during replication.