Interestingly, type III and IV MAbs also bound to the HR2 domain and display neutralizing and cell-cell fusion inhibition properties. virus-cell membrane fusion event is an essential step in the entry process of all enveloped animal viruses, including important human pathogens such as influenza virus, human immunodeficiency virus (HIV) (8,23), and the GPR120 modulator 1 newly emerged severe acute respiratory syndrome coronavirus (SARS-CoV) (9). Following the binding to their receptors on the cell surface, virus-encoded membrane fusion proteins mediate the fusion process. In many but not all cases, the viral fusion proteins are proteolytically processed by host proteases into 2 subunits that remain closely associated with each other: a surface subunit with a receptor-binding site and a transmembrane subunit with a fusion peptide consisting of two or more heptad repeat domains. Upon interaction of the fusion protein with a cellular receptor, the buried fusion peptide is exposed and inserted into the membrane of the target cell. A series of conformational changes trigger virus-cell fusion activity (9) and lead to the unloading of the viral genome into cells. Additionally, many viral fusion proteins also induce cell-cell fusion, i.e., the formation of multinucleated syncytia, facilitating the rapid spread of virus infection. The spike (S) protein of coronaviruses is responsible for receptor binding and membrane fusion. It shares similarity with class I virus fusion proteins (2,3). Typically, it is a type GPR120 modulator 1 I integral membrane protein, which is GPR120 modulator 1 N-glycosylated and trimerized in the endoplasmic reticulum. The N-terminal S1 protein contains the receptor-binding site (10,18,22,34). The C-terminal S2 protein is a fusion subunit and anchors on the viral envelope through a transmembrane domain. The S2 protein ectodomain contains two 4,3 hydrophobic heptad repeats (HR1 and HR2) and a putative, internal fusion peptide (3,23). For GPR120 modulator 1 the SARS-CoV S protein, the HR2 is located adjacent to the transmembrane domain, whereas the HR1 is 140 to 170 residues upstream of the HR2. Crystallographic, biophysical, and biochemical analysis of the fusion core of SARS-CoV S protein (2,12,19,27,30,35) and other class I fusion proteins (8,25) supports a model of membrane fusion probably adopted by these enveloped viruses. After the attachment of the receptor-binding subunit to the receptor, the HR1 and HR2 domains in the membrane fusion subunit interact with each other and form a six-helix bundle, a complex consisting of a homotrimeric HR1 coiled coil surrounded by three HR2 helices. The spacer domain (or link, or interhelical domain) between HR1 and HR2 forms a loop and reverses the direction of the polypeptide chain so that the HR2 helices pack against the HR1 coiled coil in an antiparallel manner. This conformational change results in a close apposition of the fusion peptide, already exposed and inserted into the target cellular membrane, with the viral transmembrane domain, leading to virus-cell membrane fusion. Obviously, the functional domains involved in membrane fusion are attractive targets for the discovery of viral entry inhibitors. Peptides GPR120 modulator 1 derived from HR1 or HR2 can inhibit infection as shown for coronaviruses and many other viruses, presumably by interfering with the formation of the six-helix bundle and inhibiting the initiation of membrane fusion. This strategy has been successfully used in the development of inhibitors for HIV infection (20). Potentially, monoclonal antibodies (MAbs) targeting fusion subunits might also inhibit the fusion in the same manner (21). For example, neutralizing MAbs against gp41 of HIV are likely to target fusion intermediates or epitopes that are exposed following receptor interactions (39,41). The S protein contains the determinants for host Rabbit polyclonal to CD47 specificity, cell tropism, and pathogenesis. The S protein is also able to stimulate humoral and cellular immune responses (1,6,7,13,32), and therefore, it is one of the major targets for the development of vaccine candidates. Identification and characterization of neutralizing epitopes on S of SARS-CoV could provide useful leads to the development of efficacious vaccines (11,26,31,37,38). In our previous study.
