Having shown that UL44 can be modified by SUMO-1, we wondered whether it could be conjugated also to SUMO-2 and SUMO-3. Thus, the in vitro sumoylation system was applied to purified 6His-UL44 in the presence of sumoylation enzyme components and activated forms of SUMO-2 or SUMO-3. In these assays, the RanGTPase-activating protein RanGAP1 [54] was used as a positive control and as previously observed [55] , was modified in the presence of SUMO-2 and SUMO-3 (data not shown). As shown in Fig. 2B , UL44 was also modified by either of the two SUMO peptides.