Crystalline Antigen from the Influenza Virus Envelope

Abstract

THE envelope of influenza viruses contains two morphologically and chemically distinct glycoproteins, a haemagglu-tinin and a neuraminidase, which recent experiments have indicated¹ may be removed from the virion by treatment with the protease, bromelain. In an investigation of the mechanism of this phenomenon it was found that although the neuraminidase protein was completely degraded during proteolysis, the isolated haemagglutinin protein could be recovered almost unchanged from the incubation mixture. We report the subsequent purification and crystallization of the haemagglutinin and its preliminary chemical and antigenic characterization. The X-31 strain of A2/Hong Kong/68² was used in most of the experiments to be described. Viruses were grown in embryonated eggs, purified as described by Skehel and Schild³, and digested with bromelain as described by Compans et al.¹. The smooth-surfaced virus particles which resulted were removed from the incubation mixture by centrifuging for 60 min at 100,000g and purified by sucrose density gradient centrifugation (20–60% sucrose in phosphate buffered saline (PBS) (100,000g, 16 h). The morphology of these particles in comparison with intact virus is shown in Fig. 1. It is clear that all of the envelope spike proteins were removed during proteolytic digestion; in agreement with these observations the shaved particles were completely devoid of the haemagglutinating and neuraminidase activities of the virus envelope proteins. Polyacrylamide gel electrophoretic analyses of the polypeptide composition of the particles also indicated that they contained only four of the seven polypeptides of the virus⁴ in the same proportions as these were present in the intact virus particles (Fig. 2b and c). Both haemagglu-tination and neuraminidase activities were completely destroyed during protease treatment. Electrophoretic analyses of the proteins present in the protease incubation supernatant after removal of the subviral particles indicated, however (Fig. 2a), that the major components of this mixture were two polypeptides of similar mobility to the haemagglutinin polypeptides previously identified³. Thus, the mobility of the larger polypeptide of the supernatant was identical to that of the larger haemagglutinin component and the mobility of the smaller polypeptide indicated a molecular weight difference of approximately 3,000 between this and the smaller haemagglutinin polypeptide.