Iodination of influenza virus by two methods followed by analysis of the labeled polypeptides using polyacrylamide gel electrophoresis under reducing and nonreducing conditions has been used to investigate the structure of the virus. Lactoperoxidase (MW ∼ 78,000) catalysed the iodination of the surface proteins of intact virions, but not the ribonucleoprotein or the recently described, low molecular weight protein (now designated the internal protein). By contrast, chloramine T (MW = 282) oxidation resulted in the labeling of all the viral proteins, but under certain conditions the internal protein and the hemagglutinin were labeled much more rapidly than the ribonucleoprotein of intact virus. Results from these two experimental approaches suggest that the internal protein is situated between the nucleocapsid and the spikes of the envelope. The hemagglutinin from influenza virus strain A/Bel grown in eggs was found to be composed of two glycopeptides with molecular weights in the region of 65,000 and 23,000 when analysed by polyacrylamide gel electrophoresis under stringent reducing conditions. The larger glycopeptide contains a considerably higher proportion of carbohydrate than the smaller one. Iodination of these components from virus in different states of dissociation has revealed their relation to each other, and to the viral membrane, and provided some insight into the conformation of the hemagglutinin. It would now appear that each hemagglutinin "spike" is a dimer made up of two, noncovalently associated glycoprotein molecules, each of which is composed of two different glycopeptides bound covalently by disulfide bonds. The hemagglutinin of Bel virus grown in cultured cells could not be clearly dissociated into two components, an observation suggesting a fundamental difference in the structure of the hemagglutinins from these sources.
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