Structural requirements for hemolytic activity of F-glycoprotein of HVJ (Sendai virus) studied by proteolytic dissection.

The structure-function relationship of F and HN glycoproteins of HVJ were studied by proteolytic dissection. Three types of effects on the biological activity and structure of the virus particles were observed. First type of effect is preferential inactivation of biological activities related to F glycoprotein, such as hemolytic and cell fusion-inducing activities. Among enzymes which exert such effects, trypsin split F1 subunit to F1a (32,000 daltons) and F1b (19,000 daltons). By N-terminal determination, F1a was found to be derived from the N-terminal segment of F1, whereas F1b seems to correspond with the C-terminal segment of F1. Chymotrypsin and thermolysin digestion resulted in decreases in molecular weight of F1 subunit by about 3,500 daltons and 2,500 daltons, respectively. This splitting was found to occur near the N-terminus of F1, since new N-terminal amino acids were identified from the modified F1's. The second type of effect is characterized by specific splitting (for example, by a Staphylococcal proteases) of HN glycoprotein without affecting F protein. The third type has no apparent effect on the biological activities of the virion, although slight structural change of F glycoprotein was noted in some case. Exposure of the N-terminal segment of F1 to the surrounding aqueous medium despite its highly hydrophobic nature is shown by its easy splitting by aminopeptidase M, chymotrypsin and thermolysin. Based on these and previously published results, we hypothesize direct interaction of the hydrophobic segment with the lipid bilayers of the target cell membrane as an important step in fusion reactions between the viral envelope and plasma membranes.