Green fluorescent protein (GFP)-tagged mouse hepatitis coronavirus nonstructural protein 4 (nsp4) was shown to localize to the endoplasmic reticulum (ER) and to be recruited to the coronavirus replicative structures. Fluorescence loss in photobleaching and fluorescence recovery after photobleaching experiments demonstrated that while the membranes of the ER are continuous with those harboring the replicative structures, the mobility of nsp4 at the latter structures is relatively restricted. In agreement with that observation, nsp4 was shown to be engaged in homotypic and heterotypic interactions, the latter with nsp3 and nsp6. In addition, the coexpression of nsp4 with nsp3 affected the subcellular localization of the two proteins.