Y2H screenings and interaction assays. Growth media and standard methods for manipulating yeast cells were as described [37] . Saccharomyces cerevisiae strain L40 was transformed [38] with the bait plasmid pBTMK-UL44 and subsequently with either of two cDNA libraries fused to GAD (see Supplementary Material and Methods in Text S1). Primary transformants were selected for growth on -His-Leu-Trp dropout plates. His + colonies were thereafter analyzed for b-galactosidase activity by filter lift experiments [39] . Double positive clones were subjected to another cycle of screening (for further details see Supplementary Material and Methods in Text S1). cDNA inserts of interactor plasmids were sequenced and analyzed with BLAST (www.ncbi/ blastn). To quantify b-gal expression, the method of Breeden and Nasmyth [40] was used.