The fusion-related hydrophobic domain of Sendai F protein can be moved through the cytoplasmic membrane of Escherichia coli.

Recent work on a prokaryotic membrane protein, gene III protein (pIII) of coliphage f1, showed that polypeptide segments of sufficient hydrophobicity functioned to stop transfer of the polypeptide across the cell membrane: strings of 16 or more hydrophobic amino acids sufficed. A fusion-related hydrophobic domain (FRHD) of Sendai F protein, a sequence of 26 consecutive uncharged residues, has been implicated in the fusion of the viral membrane envelope and the target-cell membrane through a hydrophobic interaction. As it is located on the exterior of the viral membrane, this sequence must be transferred across the host-cell membrane during synthesis. We have inserted either the FRHD or the F protein membrane anchor (the COOH-terminal region of the F protein) into an internal site of a secreted pIII, which lacks its natural membrane anchor. These two hydrophobic sequences behave in the bacteria just as they do in their natural eukaryotic cell host. The F protein membrane anchor functions to stop transfer, conferring a membrane-spanning topology to the F-pIII hybrid protein; however, the FRHD is moved through the cytoplasmic membrane and derivatives carrying this sequence are secreted to the periplasm. We discuss how the FRHD is compatible with passage through the membrane and yet is still able to mediate membrane fusion through a presumed hydrophobic interaction.