Leucine at position 278 of the AIK-C measles virus vaccine strain fusion protein is responsible for reduced syncytium formation

The live measles virus (MV) vaccine strain AIK-C was attenuated from the wi ld-type strain Edmonston by plaque purification at 33 degreesC. Strain AIK- C grew well at 33 degreesC with a mixture of small and medium-sized plaques in Vero cells, but did not grow well at 40 degreesC. To investigate fusion inducibility, expression plasmids for the fusion (F) and haemagglutinin (H ) protein regions of MV strains AIK-C (pAlK-FO1 and pAIK-H) and Edmonston ( pEdm-F and pEdm-H) were constructed. pEdm-F induced extensive cell fusion i n B95a and Vero cells under the control of T7 RNA polymerase, whereas a sha rp reduction in syncytium formation was observed when pAIK-FO1 was used. Si x amino acid differences were determined between pAIK-FO1 and pEdm-F. Direc t sequencing showed that the seed strain AIK-C contained either Leu or Phe at position 278 of the F protein. Experiments using recombinant F protein p lasmids demonstrated that those with Leu at position 278 induced poor syncy tium formation, while those with Phe at position 278 (Edmonston type) induc ed extensive cell fusion. Replacement of Phe with Leu at position 278 of pE dm-F reduced fusion-inducing capability. A full-length infectious clone of AIK-C with Leu at position 278 of the F protein was constructed. The rescue d virus produced small plaques in Vero cells. However, the same rescued vir us with Phe at position 278 produced large plaques. It was concluded that L eu at position 278 of the F protein of the MV vaccine strain AIK-C is respo nsible for the formation of small plaques.