Encephalomyocarditis virus and Theilovirus are species in the Cardiovirus genus of the Picornaviridae family. For all cardioviruses, the viral
polyprotein is initiated with a short Leader (L)
protein unique to this genus. The nuclear magnetic resonance (NMR) structure of LE from encephalomyocarditis virus (EMCV) has been determined. The
protein has an NH2-proximal CHCC zinc finger, a central linker, and a contiguous, highly acidic motif. The theiloviruses encode the same domains, with one or two additional, COOH-proximal domains, characteristic of the human Saffold viruses (SafV) and Theiler's murine
encephalomyelitis viruses (TMEV), respectively. The expression of a cardiovirus L, in recombinant form, or during
infection/transfection, triggers an extensive, cell-dependent, antihost phosphorylation cascade, targeting
nucleoporins (Nups) that form the hydrophobic core of nuclear pore complexes (NPC). The consequent inhibition of active nucleocytoplasmic trafficking is potent and prevents the host from mounting an effective
antiviral response. For this inhibition, the L
proteins themselves must be phosphorylated. In cells (extracts or recombinant form), LE was shown to be phosphorylated at Thr47 and Tyr41. The first reaction (Thr47), catalyzed by
casein kinase 2 (CK2), is an obligatory precedent to the second event (Tyr41), catalyzed by
spleen tyrosine kinase (Syk). Site mutations in LE, or
kinase-specific inhibitors, prevented LE phosphorylation and subsequent Nup phosphorylation. Parallel experiments with LS (SafV-2) and LT (TMEV BeAn)
proteins confirmed the general cardiovirus requirement for L phosphorylation, but CK2 was not the culpable
kinase. It is likely that LS and LT are both activated by alternative
kinases in different cell types, probably reactive within the Theilo-specific domains. IMPORTANCE An understanding of the diverse methods used by viruses to interfere with cellular processes is important because they can teach us how to control
virus infections. This report shows how viruses in the same genus use different cellular
enzymes to phosphorylate their
proteins. If these processes are interfered with, the viruses are severely disabled.