Particles of the Puerto Rico isolate of cowpea mosaic virus (CPMV-PR), which belongs in the severe subgroup, had one electrophoretic form when analysed by 2.5% polyacrylamide gel electrophoresis and zone electrophoresis in sucrose gradients at pH 7.8, 7.5, and 5.5. The electrophoretic properties of CPMV-PR did not vary with time after inoculation but the virus could be converted to a slower migrating form by treatment with trypsin. The partial conversion by trypsin was accompanied by and was possibly the result of proteolytic conversion of the S protein subunit to a lower molecular mass form. Analysis of proteins of untreated CPMV-PR suggested that the S subunit of the virus could also be converted by plant proteases to a smaller form without any change in the particle net surface charge, while treatment with trypsin affected those S subunits not previously converted by plant proteases, by converting them to a form slightly larger and with a greater positive charge than the S subunit produced by action of the plant proteases. This hypothesis accounts both for the decrease in electrophoretic mobility and for the fact that the conversion was only partial.


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