Successful viruses have evolved superior strategies to escape host defenses or 
exploit host biological pathways. Most of the viral immediate-early (ie) genes 
are essential for viral infection and depend solely on host proteins; however, 
the molecular mechanisms are poorly understood. In this study, we focused on the 
modification of viral IE proteins by the crayfish small ubiquitin-related 
modifier (SUMO) and investigated the role of SUMOylation during the viral life 
cycle. SUMO and SUMO ubiquitin-conjugating enzyme 9 (UBC9) involved in 
SUMOylation were identified in red swamp crayfish (Procambarus clarkii). Both 
SUMO and UBC9 were upregulated in crayfish challenged with white spot syndrome 
virus (WSSV). Replication of WSSV genes increased in crayfish injected with 
recombinant SUMO or UBC9, but injection of mutant SUMO or UBC9 protein had no 
effect. Subsequently, we analyzed the mechanism by which crayfish SUMOylation 
facilitates WSSV replication. Crayfish UBC9 bound to all three WSSV IE proteins 
tested, and one of these IE proteins (WSV051) was covalently modified by SUMO in 
vitro. The expression of viral ie genes was affected and that of late genes was 
significantly inhibited in UBC9-silenced or SUMO-silenced crayfish, and the 
inhibition effect was rescued by injection of recombinant SUMO or UBC9. The 
results of this study demonstrate that viral IE proteins can be modified by 
crayfish SUMOylation, prompt the expression of viral genes, and ultimately 
benefit WSSV replication. Understanding of the mechanisms by which viruses 
exploit host components will greatly improve our knowledge of the virus-host 
"arms race" and contribute to the development of novel methods against virulent 
viruses.