A hemagglutination-inhibitory mucoprotein from human urine has been studied with the electron microscope. It consists of filaments, with diameters of 40 to > 240 A, composed of smaller fibrils. In the two-dimensional projection of the electron micrographs, the single fibrils often show a zig-zag course with a periodicity of 100 to 140 A; the single branch of a zig-zag measures about 60 A in length and either 20 or 40 A in width. Still thinner fibrillar elements are observable with diameters of 10 A or less. In three-dimensional aspect, the zig-zag structure might be a helix. The fibril-bundle (or filament) reveals a complicated configuration. Heat treatment at 70°C shows some indication of denaturation (e.g. filaments are shorter), whereas at 80°C almost complete degradation of the protein into individual zig-zag elements or smaller pieces is attained. The interaction between influenza virus particles and inhibitory mucoprotein consists of the attachment of a fiber molecule to the virus projections at several sites and frequently on more than one virus particle.

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