Canine distemper virus and measles virus fusion glycoprotein trimers: Partial membrane-proximal ectodomain cleavage enhances function

The trimeric fusion (F) glycoproteins of morbilliviruses are activated by furin cleavage of the precursor F₀ into the F₁ and F ₂ subunits. Here we show that an additional membrane-proximal cleavage occurs and modulates F protein function. We initially observed that the ectodomain of approximately one in three measles virus (MV) F proteins is cleaved proximal to the membrane. Processing occurs after cleavage activation of the precursor F₀ into the F₁ and F₂ subunits, producing F_1a and F_1b fragments that are incorporated in viral particles. We also detected the F_1b fragment, including the transmembrane domain and cytoplasmic tail, in cells expressing the canine distemper virus (CDV) or mumps virus F protein. Six membrane-proximal amino acids are necessary for efficient CDV F_1a/b cleavage. These six amino acids can be exchanged with the corresponding MV F protein residues of different sequence without compromising function. Thus, structural elements of different sequence are functionally exchangeable. Finally, we showed that the alteration of a block of membrane-proximal amino acids results in diminished fusion activity in the context of a recombinant CDV. We envisage that selective loss of the membrane anchor in the external subunits of circularly arranged F protein trimers may disengage them from pulling the membrane centrifugally, thereby facilitating fusion pore formation.