Purification of the fusion protein of Sendai virus: analysis of the NH2-terminal sequence generated during precursor activation

The two glycoproteins of Sendai virus, the hemagglutinin-neuraminidase and the fusion protein (F), were separated and purified by affinity chromatography on a Lens culinaris lectin-Sepharose column. F was shown to consist of two disulfide-bonded glycopolypeptide chains, F1 and F2, of molecular weights 51,000 and 11,000, each of which contained 15% carbohydrate by weight. Amino-terminal sequence analysis showed that F2 was blocked and that the hydrophobic sequence NH2-Phe-Phe-Gly-Ala-Val-Ile-Gly-Ile-Ile-Ala-Leu-Gly-Pro-Ala-Thr- was at the amino terminus of F1. This sequence shows identity at six positions with the hydrophobic amino-terminal sequence of the smaller glycopolypeptide chain, HA2, of the hemagglutinin of influenza virus. Both F1 and HA2 are formed by proteolytic cleavage of precursor glycoproteins (Fo, Sendai virus; HAo, influenza virus). Since these cleavages confer infectivity upon both Sendai and influenza viruses and the ability to induce cell-to-cell fusion upon Sendai virus, the hydrophobic NH2-terminal sequences on F1 and HA2 may play a role in fusion of viral and host-cell membranes.