A protein G fragment from the Salmonid viral hemorrhagic septicemia rhabdovirus induces cell-to-cell fusion and membrane phosphatidylserine translocation at low pH

The fusion-related properties of segments p9, p3, p4, and p9 + p2 surroundi ng the p2 phospholipid-binding domain of the protein G (pG) of the salmonid rhabdovirus of viral hemorrhagic septicemia (VHS) (Nunez, E., Fernandez, A . M., Estepa, A., Gonzalez-Ros, J. M., Gavilanes, F., and Coll, J. M. (1998 ) Virology 243,322-330; Estepa, A., and Coll, J. M. (1996) Virology 216, 60 -70), have been studied at neutral and fusion (low) pH values by using its derived peptides. Cell-to-cell fusion, translocation of phosphatidylserine, and inhibition of fusion of pG-transfected cells defined the p9 + p2 (frag ment 11, sequence 56-110) as a fragment with higher specific activity for a nionic phospholipid aggregation than the previously reported p2. While frag ment 11, p2, and p3 showed interactions with anionic phospholipids, p9 and p4 showed no interactions with any phospholipids. When added to a cell mono layer model at low pH, fragment 11 induced pH-dependent cell-to-cell fusion and translocated phosphatidylserine from the inner to the outer leaflet of the membrane. At low pH and in the presence of anionic phospholipids, frag ment 11 showed more than 80% beta -sheet conformation (IR and CD spectrosco pies). Finally, anti-fragment 11 antibodies inhibited low pH-dependent pG-t ransfected cell-to-cell fusion. All of the data support the conclusion that fragment 11 is a primary determinant of some of the viral cell fusion even ts in VHSV.