The spatial organization of metastable paramyxovirus fusion (F) and attachment glycoprotein heterooligomers is largely unknown. To further elucidate the organization of functional fusion complexes of measles virus (MeV), an archetype of the paramyxovirus family, we subjected central predictions of alternative docking models to experimental testing using three distinct approaches. Carbohydrate shielding through engineered N-glycans indicates close proximity of a membrane-distal, but not membrane-proximal, section of the MeV attachment (H) protein stalk domain to F. Directed mutagenesis of this section identified residues 111, 114, and 118 as modulators of avidity of glycoprotein interactions and determinants of F triggering. Stalk-length variation through deletion or insertion of HR elements at positions flanking this section demonstrates that the location of the stalk segment containing these residues cannot be altered in functional fusion complexes. In contrast, increasing the distance between the H head domains harboring the receptor binding sites and this section through insertion of structurally rigid ␣-helical domains with a pitch of up to approximately 75 Å downstream of stalk position 118 partially maintains functionality in transient expression assays and supports efficient growth of recombinant virions. In aggregate, these findings argue against specific protein-protein contacts between the H head and F head domains but instead support a docking model that is characterized by short-range contacts between the prefusion F head and the attachment protein stalk, possibly involving H residues 111, 114, and 118, and extension of the head domain of the attachment protein above prefusion F.Paramyxoviruses infect cells through fusion of the viral envelope with target cell membranes. For all members of the Paramyxovirinae subfamily, this involves the concerted action of two envelope glycoproteins, the fusion (F) and attachment (H, HN, or G, depending on the Paramyxovirinae genus) proteins. Both proteins feature short lumenal tails, a single transmembrane domain, and large ectodomains. The F protein, in type I orientation, forms homotrimers, while homodimers or homotetramers have been suggested as functional units for attachment proteins of different Paramyxovirinae subfamily members (7,14,28,41,49,50,66). For entry, upon receptor binding, the attachment protein is considered to initiate a series of conformational rearrangements in the metastable prefusion F protein (15, 77), which ultimately brings together transmembrane domains and fusion peptides and, thus, donor and target membranes (3,32,45,53,80).Multiple studies have demonstrated that specific interactions between compatible F and attachment proteins of paramyxovirinae are imperative for the formation of functional fusion complexes (6,29,36,42,43,56,75). However, the molecular nature of these interactions and the spatial organization of functional glycoprotein hetero-oligomers remain largely unknown. Individual ectodomain and partial ectodomain crystal stru...

Kaposi's sarcoma (KS) is an endothelial cell-derived tumor. Investigations of the molecular mechanisms of KS pathogenesis and the identification of drugs for treatment of KS depend critically on valid cell-culture models. Two major immortalized cell lines are available for KS research. Recently, the KS cell line KS Y-1 has been shown to be cross-contaminated with the T24 urinary bladder cancer cell line (ATCC HTB-4). Here, we show by short tandem repeat profiling that the second KS cell line, SLK, is indistinguishable from the clear-cell renal-cell carcinoma cell line Caki-1. Immunocytochemical detection of cytokeratin expression confirmed the epithelial-cell origin of SLK cells. Our findings indicate that SLK cells are not of endothelial origin and should not be used in future studies as a model for KS-derived endothelial tumor cells. We suggest that in the future, more attention needs to be paid to the authenticity of cells in lines derived from human tissues.

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