Identification of biological activities of paramyxovirus glycoproteins. Activation of cell fusion, hemolysis, and infectivity by proteolytic cleavage of an inactive precursor protein of Sendai virus

Abstract

The glycoproteins of Sendai virus have been isolated by a procedure involving extraction with the nonionic detergent Triton X-100 and affinity chromatography on fetuin-Sepharose. The largest Sendai virus glycoprotein (MW ~69,000) possesses both hemagglutinating and neuraminidase activities, and has been designated HN. Virions grown in MDBK cells or in the allantoic sac of the chick embryo contain similar amounts of the HN glycoprotein, but differ in their content of the other glycoproteins. Virions grown in MDBK cells contain a large amount of a glycoprotein designated F 0 (MW ~65,000). This glycoprotein is a precursor of a smaller virion glycoprotein, F (MW ~53,000) which is present in only small amount in MDBK cellgrown virions. F 0 can be cleaved to yield F by treatment of virions with trypsin in vitro. Sendai virions grown in the chick embryo lack the precursor protein F 0, but contain a large amount of F; in this case, proteolytic cleavage of F 0 to yield F occurs in ovo. The Sendai virions grown in MDBK cells, which are deficient in the small glycoprotein F, lack hemolyzing and cell-fusing activities and cannot infect MDBK cells. Virions grown in the chick embryo, which contain much F, possess both these activities and can infect MDBK cells as well as the chick embryo. MDBK cell-grown virions acquire hemolyzing and cell-fusing activities and become infective for MDBK cells when the precursor glycoprotein F 0 is cleaved in vitro to yield F. The results indicate that the small glycoprotein of paramyxoviruses is biologically active and is involved in virus-induced hemolysis, cell fusion, and the initiation of infection. The precise mechanism by which this glycoprotein participates in these reactions remains to be determined, but is now amenable to experimentation. The precursor of this glycoprotein is biologically inactive, but is incorporated into virions grown in some host cells; it may be activated by proteolytic cleavage either in vivo or in vitro. The present results provide a biochemical basis for previously observed host-dependent variation in infectivity, and in hemolysis and cell-fusion induced by paramyxoviruses.