The proteome of human cytomegalovirus virions and dense bodies is conserved across different strains.

The morphogenesis of human cytomegalovirus (HCMV) particles is incompletely understood. Analysis of the protein composition of HCMV virions and subviral dense bodies (DBs) by mass spectrometry provides valuable information to increase our knowledge about viral morphogenesis. Here we addressed the viral proteome of virions and DBs from two fibroblast-passaged isolates and the widely used endotheliotropic TB4-BAC40 strain of HCMV. The results show a striking concordance of the particle proteomes of different strains. One surprising finding was that only low levels of gpUL128-131A were found in TB40-BAC4 virions. These three proteins, together with gH and gL, form a protein complex that is critical for the endothelial cell tropism of that strain. This indicates that either few molecules of that complex per virion or a small fraction of pentamer-positive virions suffice to retain the tropism. Furthermore, using a pp65-deficient variant of TB40-BAC4, we confirm our previous finding that the major tegument protein serves as a scaffold to support the upload of a fraction of the outer tegument proteins into particles. The results demonstrate that HCMV particle morphogenesis is an orchestrated process that leads to the formation of particles with a largely strain-independent protein composition.