Non-structural proteins 2 and 3 interact to modify host cell membranes during the formation of the arterivirus replication complex

The replicase polyproteins of equine arteritis virus (EAV; family Arteriviridae, order Nidovirales) are processed by three viral proteases to yield 12 non-structural proteins (nsps). The nsp2 and nsp3 cleavage products have previously been found to interact, a property that allows nsp2 to act as a co-factor in the processing of the downstream part of the polyprotein by the nsp4 protease. Remarkably, upon infection of Vero cells, but not of BHK-21 or RK-13 cells, EAV nsp2 is now shown to be subject to an additional, internal, cleavage. In Vero cells, approximately 50% of nsp2 (61 kDa) was cleaved into an 18 kDa N-terminal part and a 44 kDa C-terminal part, most likely by a host cell protease that is absent in BHK-21 and RK-13 cells. Although the functional consequences of this additional processing step are unknown, the experiments in Vero cells revealed that the C-terminal part of nsp2 interacts with nsp3. Most EAV nsps localize to virus-induced double-membrane structures in the perinuclear region of the infected cell, where virus RNA synthesis takes place. It is now shown that, in an expression system, the co-expression of nsp2 and nsp3 is both necessary and sufficient to induce the formation of double-membrane structures that strikingly resemble those found in infected cells. Thus, the nsp2 and nsp3 cleavage products play a crucial role in two processes that are common to positive-strand RNA viruses that replicate in mammalian cells: controlled proteolysis of replicase precursors and membrane association of the virus replication complex.