Site directed mutagenesis of the murine coronavirus spike protein. Effects on fusion

Mutations were introduced in the transmembrane region of the spike protein of the murine coronavirus A59. The maturation of these mutant S proteins was not affected, they were all expressed at the cell surface, and became acylated, however some mutant S proteins did not induce cell-to-cell fusion. An I-->K change in the middle of the predicted transmembrane (TM) anchor and mutation of the first three cysteine residues of the TM domain resulted in a fusion-negative phenotype. We propose a model by which these data can be explained.