Paramyxoviruses, including the emerging lethal human Nipah virus (NiV) and the avian Newcastle disease virus (NDV), enter host cells through fusion of the viral and target cell membranes. containing the NDV HN receptor binding regions and the NiV G stalk domain require Rabbit Polyclonal to Cytochrome P450 2S1 a specific sequence at the connection between the head and the stalk to activate NiV F for fusion. MK 0893 Our findings are consistent with a general mechanism of paramyxovirus fusion activation in which the stalk domain of the receptor binding protein is responsible for F activation and a specific connecting region between the receptor binding globular head and the fusion-activating stalk domain is required for transmitting the fusion signal. INTRODUCTION The entry of enveloped viruses into host cells requires fusion of the viral and cell membranes. Viral fusion is driven by specialized fusion proteins that bring the viral and host membranes in close apposition to form a fusion pore (reviewed previously [1,C6]). The trigger that initiates a series of conformational changes in F leading to membrane fusion differs depending on the pathway that the virus uses to enter the cell, i.e., whether fusion occurs at neutral pH at the surface or at low pH in the endosome. For paramyxoviruses, the F protein is activated when the adjacent receptor binding protein binds to its receptor on host cell and initiates the fusion process (7). Once activation occurs, the fusion protein undergoes a coordinated series of conformational changes that progress toward the most stable form of the protein and promote membrane fusion (reviewed in references 8 and 9). The role of the receptor binding protein in this process is critical (10,C15). Paramyxoviruses possess envelope proteins that provide a receptor binding function and, depending on the specific paramyxovirus family member, a receptor cleaving (neuraminidase) activity. A recently identified function of the receptor binding protein of human parainfluenza virus 3 (HPIV3), which may apply to other paramyxoviruses (16), is to stabilize the fusion protein and prevent its activation until the virus engages receptor (17). Most paramyxovirus receptor binding proteins studied to date also serve the critical function of activating the fusion protein (F) upon receptor engagement. The receptor binding proteins possess a membrane distal globular head domain that engages the receptor and a membrane proximal stalk that confers specificity toward the homologous F protein. For Newcastle disease virus (NDV), the envelope protein hemagglutinin-neuraminidase MK 0893 (HN) contains both receptor binding and neuraminidase activities. When bound to receptor, HN triggers F to undergo conformational changes that lead to membrane fusion (7, 18,C20). HN is a type II membrane protein with a cytoplasmic domain, a membrane-spanning region, a stalk region, and a globular head that interacts with sialic acid receptors. Structural analysis of the HNs from avian NDV (21, 22), HPIV3 (23), and simian virus 5 (or parainfluenza virus type 5 [PIV5]) (24) has identified the locations of the primary binding/neuraminidase active-site residues on the globular head of the molecule, as well as several key structural elements that are required for the fusion-triggering function of HN (7, 18,C20). The analyses of NDV revealed two sialic acid binding regions, sites I and II, on HN. We previously reported that site II can be activated for receptor binding by small molecules (e.g., zanamivir) that occupy site I (25), and this finding was supported by recent analysis of a series of NDV HN mutants (25,C27). We recently described a chimeric protein consisting of the globular head of NDV HN and the stalk domain of NiV G that activates NiV F, meaning that the head of a heterotypic paramyxovirus can signal F through a homotypic stalk. Activation of site II of the receptor binding protein is a determinant for fusion MK 0893 activation (27, 28). We now explore the hypothesis that the connecting region between the stalk domain and the globular head of the receptor binding protein plays a pivotal role in fusion promotion, whether the fusion protein is homotypic or heterotypic with respect to the globular head. Specific residues between the stalk and globular domains of the receptor binding protein are required for efficient triggering of NiV F, and alterations in this connecting region prevent the globular head from activating the stalk domain. Our results are consistent with a unified mechanism of fusion activation for paramyxoviruses, in which the globular domain of the receptor binding protein transmits the fusion signal to the F protein through the stalk domain of the binding protein. MATERIALS AND METHODS Cell cultures. 293T (human kidney epithelial cells) were grown in Dulbecco modified Eagle medium (Gibco) supplemented with 10% fetal bovine serum and antibiotics in a humidified incubator supplemented with 5% CO2..