Intramolecular complementation of measles virus fusion protein stability confers cell–cell fusion activity at 37 °C

• Stabilized MV F protein by the N465H mutation in the HR-B domain lost fusion activity.
• Destabilized MV F protein by a mutation in the DIII domain decreased fusion activity.
• Due to stability complementation, doubly mutated F protein restored fusion activity.

The fusion (F) protein of measles virus mediates membrane fusion. In this study, we investigated the molecular basis of the cell–cell fusion activity of the F protein. The N465H substitution in the heptad repeat B domain of the stalk region of the F protein eliminates this activity, but an additional mutation in the DIII domain of the head region – N183D, F217L, P219S, I225T or G240R – restores cell–cell fusion. Thermodynamically stabilized by the N465H substitution, the F protein required elevated temperature as high as 40 °C to promote cell–cell fusion, whereas all five DIII mutations caused destabilization of the F protein allowing the highest fusion activity at 30 °C. Stability complementation between the two domains conferred an efficient cell–cell fusion activity on the F protein at 37 °C.