The UL130 gene is one of the major determinants of endothelial cell (EC) tropism of human cytomegalovirus (HCMV). In order to define functionally important peptides within this protein, we have performed a chargecluster-to-alanine (CCTA) mutational scanning of UL130 in the genetic background of a bacterial artificial chromosome-cloned endotheliotropic HCMV strain. A total of 10 charge clusters were defined, and in each of them two or three charged amino acids were replaced with alanines. While the six N-terminal clusters were phenotypically irrelevant, mutation of the four C-terminal clusters each caused a reduction of EC tropism. The importance of this protein domain was further emphasized by the fact that the C-terminal pentapeptide PNLIV was essential for infection of ECs, and the cell tropism could not be rescued by a scrambled version of this sequence. We conclude that the C terminus of the UL130 protein serves an important function for infection of ECs by HCMV. This makes UL130 a promising molecular target for antiviral strategies, e.g., the development of antiviral peptides.Human cytomegalovirus (HCMV) is a widespread betaherpesvirus that causes lifelong persistent infections with occasional reactivations. While HCMV infection is usually clinically unapparent in the immunocompetent host, it can cause severe disseminated infections under conditions of immunosuppression, with manifestations in the lung, retina, and gastrointestinal tract, among others (12). Various cell types support viral replication, including epithelial cells and endothelial cells (ECs), smooth muscle cells, fibroblasts, and cells of hematopoietic origin (13,14,18,19,25,26,37). Among these target cells, endothelial cells are assumed to contribute particularly to hematogenous dissemination of HCMV (24).While recent clinical HCMV isolates are characterized by this broad cell tropism, the target cell range becomes restricted during long-term propagation on fibroblasts (28, 33). The underlying mechanism for this cell culture adaptation is a modulation within the viral genes UL128, UL130, and UL131A (8, 11). These three genes have been shown to be essential for infection of granulocytes, dendritic cells, epithelial cells, and endothelial cells but are dispensable for infection of fibroblasts (1, 9, 11, 34, 35). The encoded proteins pUL128, pUL130, and pUL131A were reported to form a complex with the viral glycoproteins gH and gL that is distinct from the glycoprotein complex gCIII (gH/gL/gO) (35). Whereas poorly endotheliotropic HCMV strains bear just the gH/gL/gO complex in their envelopes, highly endotheliotropic strains bear both gCIII variants: gH/gL/gO and gH/gL/pUL128-131A. Deletion of any of the three genes UL128-131A results in loss of EC tropism (11), most likely because only a complete complex of gH/gL and pUL128, pUL130, and pUL131A can efficiently function in endocytic entry in ECs (21). However, functional sites within the proteins (e.g., mediating binding to the viral complex partners or interaction with a putative cellular rece...