Reovirus FAST protein transmembrane domains function in a modular, primary sequence-independent manner to mediate cell-cell membrane fusion

The FAST proteins are a unique family of virus-encoded cell-cell membrane fusion proteins. In the absence of a cleavable N-terminal signal peptide, a single-pass transmembrane domain (TMD) functions as a reverse signal-anchor to direct the FAST proteins into the plasma membrane in an N_exo/C _cyt topology. There is little information available on the role of the FAST protein TMD in the cell-cell membrane fusion reaction. We show that in the absence of conservation in the length or primary amino acid sequence, the pl4 TMD can be functionally exchanged with the TMDs of the p10 and pl5 FAST proteins. This is not the case for chimeric pl4 proteins containing the TMDs of two different enveloped viral fusion proteins or a cellular membrane protein; such chimeric proteins were defective for both pore formation and syncytiogenesis. TMD structural features that are conserved within members of the FAST protein family presumably play direct roles in the fusion reaction. Molecular modeling suggests that the funnel-shaped architecture of the FAST protein TMDs may represent such a conserved structural and functional motif. Interestingly, although heterologous TMDs exert diverse influences on the trafficking of the pl4 FAST protein, these TMDs are capable of functioning as reverse signal-anchor sequences to direct pl4 into lipid rafts in the correct membrane topology. The FAST protein TMDs are therefore not primary determinants of type III protein topology, but they do play a direct, sequence-independent role in the membrane fusion reaction.