Interchangeability of related proteins and autonomy of function. The morphogenetic proteins of filamentous phage f1 and IKe cannot replace one another.

The filamentous phage f1 and IKe infect a common host, are structurally highly similar and exhibit 55% identity at the DNA sequence level. Based on the idea that proteins that function autonomously will be more tolerant of multiple amino acid differences than proteins that must interact with other proteins to function, the ability of four individual proteins from f1 to substitute for their IKe equivalents to promote virus assembly in vivo has been examined. The reciprocal replacements were also examined. Only the single-strand DNA binding proteins (pV) were fully interchangeable. A minor capsid protein, pIX, was unable to substitute in assembly of the heterologous phage. Two proteins required for particle assembly that are not part of the phage particle, pI and pIV, were not interchangeable, although pIVf1 stimulated formation of a very small number of IKe particles in the absence of pIVIKe. The lack of interchangeability suggests that these morphogenetic proteins do not function autonomously, but rather interact with one or more phage proteins. The ability of certain overproduced proteins to interfere with assembly of wild-type f1 or IKe forms the basis for a model that suggests that phage assembly requires an interaction between pI and pIV.