Preparations of broad bean true mosaic virus (BBTMV) contained three centrifugal components: top, middle, and bottom. These had sedimentation coefficients of 59, 95, and 116S. Top component was only found when a reducing agent was used during all stages of purification and subsequent storage. When electrophoresed on acrylamide gels, particles of BBTMV migrated as two, and sometimes three, electrophoretic forms. In new infections a slower migrating form predominated; in older infections a faster migrating form predominated. The relative migration rates of the electrophoretic forms depended on the kind of infected leaf sampled, the environmental conditions during infection, the method of purification, and the period of storage after purification. The changes in mobility of the various forms are ascribed to limited proteolysis, which increased the negative charge of the virus particle by altering the charge of the coat protein. The coat protein is made up of two polypeptides, of mol. wt. 37500 and 20000 to 24500, respectively. The change in charge resulted from a decrease in size, and possibly a change in conformation, of the smaller polypeptide.


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