Conservation of L and 3C proteinase activities across distantly related aphthoviruses

The foot-and-mouth disease virus (FMDV) leader (L) proteinase is an important virulence determinant in FMDV infections. It possesses two distinct catalytic activities: (i) C-terminal processing at the L/VP4 junction; and (ii) induction of the cleavage of translation initiation factor eIF4G, an event that inhibits cap-dependent translation in infected cells. The only other member of the Aphthovirus genus, equine rhinitis A virus (ERAV), also encodes an L protein, but this shares only 32% amino acid identity with its FMDV counterpart. Another more distantly related picornavirus, equine rhinitis B virus (ERBV), which is not classified as an aphthovirus, also encodes an L protein. Using in vitro transcription and translation analysis, we have shown that both ERAV and ERBV L proteins have C-terminal processing activity. Furthermore, expression of ERAV L, but not ERBV L, in BHK-21 cells resulted in the efficient inhibition of cap-dependent translation in these cells. We have shown that the EBAV and FMDV L proteinases induce cleavage of eIF4GI at very similar or identical positions. Interestingly, ERAV 3C also induces eIF4GI cleavage and again produces distinct products that co-migrate with those induced by FMDV 3C. The ERBV L proteinase does not induce eIF4GI cleavage, consistent with its inability to shut down cap-dependent translation. We have also shown that another unique feature of FMDV L, the stimulation of enterovirus internal ribosome entry site (IRES) activity, is also shared by the ERAV L proteinase but not by ERBV L. The functional conservation of the divergent ERAV and FMDV proteinases indicates the likelihood of a similar and important role for these enzymes in the pathogenesis of infections caused by these distantly related aphthoviruses.