Proteins. E. coli-expressed, purified GST and GST-or 6Histagged UL44 proteins were obtained as previously described [29] , with modifications (Supplementary Material and Methods in Text S1). In some preparations, samples were treated with polymin P as described [41] to eliminate residual bacterial nucleic acids. Preparation of UL44 SUMO-modified in E. coli was accomplished as described in Supplementary Material and Methods in Text S1.