The amino acid sequences of the coat proteins of four different wild strains show that a certain sequence remains always unchanged. The analysis of the coats tertiary structures confirmed this assumption. A number of mutants was generated by the use of mutagenes. Some of these mutants differ from the 158 amino acid-long coat sequence of the wild type only in one or two amino acids. The existence of a large number of just slightly differing proteins answered a number of further questions.
All mutants cross react with a serum against the wild type.
The strength of the reaction, nevertheless, does differ. Some mutants cannot be distinguished from the wild type by their serum reaction, while others display clear differences. An exchange of Pro 20 against Leu can serologically not be detected while the same exchange at position 156 shows a marked difference. These experiments proved that amino acid exchanges do only have a serological effect if the exchanged amino acid is positioned directly at the particle s surface or if the change alters the conformation of the particle surface indirectly. The changes of the polypeptide caused by mutagenes do not occur statistically, but concentrate on certain sequences.