It is relatively easy to determine that the poliovirus capsid is made up of just four proteins, and that the four capsid proteins (VP1, VP2, VP3, and VP4) are present in equimolar amounts in the capsid. Groups of five copies of each protein are arranged at each of the 12 vertices of the icosahedral capsid (see Chapters 5 and 15). If the proteins are uniformly labeled with radioactive amino acids, more radioactivity will be in each large polypeptide chain than in each small one. A gel fractionation of the radiolabeled proteins extracted from purified capsids of poliovirus is shown in Fig. 11.4.
There is much less radioactivity in the small VP4 band than in the larger protein bands; however, comparison of the bands' molecular weight with the amount of radioactivity in each reveals the equal numbers of protein molecules. Quantitative analysis of the results of a similar gel fractionation is shown in Table 11.1. Note that the ratio of sizes of VP1 to VP4, for example, is 4.5, while the ratio of radioactivity between them is also 4.5.
Capsomer (complex of all four capsid proteins)
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