Intracellular maturation of the Newcastle disease virus fusion protein is affected by strain differences in the predicted amphipathic alpha-helix adjacent to the fusion domain.

The fusion (F) glycoprotein of Newcastle disease virus (NDV) contains a predicted amphipathic alpha-helix C-terminal to its fusion domain. Of the 13 available NDV F protein sequences, only the Australia-Victoria (AV) strain alpha-helix is weakened, by the replacement of Ala159 with Thr. In this report, we demonstrate that the efficiency of cleavage and virion incorporation of the AV F protein, unlike that of other strains, is temperature sensitive. Pulse/chase experiments at 42 degrees revealed disulfide-linked aggregates containing both the F and hemagglutinin-neuraminidase glycoproteins in strain AV, but not in strain Beaudette C. Furthermore, a revertant derived from AV, whose helix-weakening Thr159 has been replaced with the consensus Ala, produced fewer F protein aggregates, confirming the structural importance of this region in maturation. In addition, a novel disulfide-defined folding intermediate of the F protein was detected.