pUL21a Interacts with the Anaphase-Promoting Complex (APC) HCMV pUL21a is a 15 kDa, highly unstable protein that is expressed with early kinetics [29] . One identified function of this protein is to facilitate efficient viral DNA synthesis [30] . However, this protein shares no significant homology with any known protein. To provide mechanistic insight into its activity, we used a proteomics approach to identify interacting partners of pUL21a during infection. We created a recombinant virus (ADgfpUL21a) in which the UL21a coding sequence was tagged with the green fluorescent protein (GFP) coding sequence. This virus grew with wild-type kinetics, and the tagged protein was fortuitously much more stable than native pUL21a [29] . A GFP tag can stabilize certain fusion proteins [31] , and made it possible to detect interacting proteins in our study. We infected fibroblasts with either ADgfpUL21a or control HCMV (ADgfp) that expressed free GFP only. At 48 hours post infection (hpi), we isolated the protein complexes from infected cells by a rapid one-step immunoaffinity purification on magnetic beads coated with GFP antibody-coupled protein A. Electrophoresis analysis revealed multiple protein bands that were specific to the pUL21a-containing sample ( Figure 1A ). We analyzed pUL21a-specific protein bands by mass spectrometry and identified the proteins depicted with arrows as APC specificity arm subunits, APC3, APC7, and APC8 (Table S1) .