Exposure of cells infected with Semliki Forest virus (SFV; ) to mildly acidic pH (5.6) results in a dramatic increase in the host cell membrane permeability due to pore formation by the virus spike proteins. Identical results were obtained when the cells were infected with two other viruses, Sindbis virus (SIN, ) and vesicular stomatitis virus (VSV, ). This permeability change could also be observed on isolated virions of SFV, SIN and VSV by measuring the influx of propidium iodide, a nucleic acid-specific fluorescent marker, into the virions. This influx was dependent on the presence of the ectodomains of the viral spikes and could be hampered by zinc ions. Furthermore, haemagglutinin, a membrane protein of influenza A virus (), expressed in Aedes cells induced a change in membrane permeability identical to that induced by the spike proteins of SFV, SIN and VSV when exposed to low pH. Thus acid-induced membrane permeability changes produced by spike proteins of three different virus families could be demonstrated in infected cells as well as in virions. Therefore, the low pH-induced pore formation by viral spike proteins seems to be more than an event specific for togaviruses and might well be an inherent property of enveloped viruses that use the endocytotic pathway to infect a cell.


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