Proteolytic cleavage of the fusion protein (F) is an important control mechanism of the biological activity of paramyxoviruses. The sequence R-R-H-K-R(112) at the cleavage site of the F protein of measles virus (MV) was altered by site-directed mutagenesis to R-N-H-N-R(112), which is not recognized by the ubiquitous cellular protease furin. When transiently expressed in cell cultures standard F protein was cleaved, whereas the mutant remained in the uncleaved form. Syncytium formation by the mutant that was analysed after coexpression with haemagglutinin protein depended on the presence of trypsin. Recombinant MV containing the mutation required trypsin activation for fusion and infectivity in cell culture. Intranasal infection of transgenic mice susceptible to MV infection (Ifnar(tm)-CD46Ge) resulted in a moderately productive infection and inflammation of the lung. In contrast to parental virus, intracerebral inoculation did not induce neural disease. The possible effects of the change in cleavage activation on tissue tropism and pathogenicity are discussed.
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