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Suppose protein A is present in a protein complex ABC. Is it possible that protein A is actually is not important for the function of protein complex ABC? Or put alternatively, can a protein X not present in ABC, yet important for the function for protein complex ABC?

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If a protein is part of a complex than it has usually an important function (otherwise it wouldn't be there). It is possible that some complexes still work when some parts are missing, but most likely not as good as they would do when they are complete. –  Chris Jul 10 at 7:10
tubulin is a structural protein and can be found unspecifically bound to a protein complex, when a binding site can not be found but thats mostly an experimentally observed (immunoprecipitation) unspecific binding and might/likely not happen in cells so most people ignore this as unspecific binding unless there is good controlled experiments. Co-factors and inhibitors tend to be important in protein complexes as they get involved in feedback mechanisms after a protein complex is formed, which is kind of similar to the second part of your question. –  Bez Jul 10 at 19:01
@Bez So protein X can be a cofactors or inhibitors? –  user677101 Jul 13 at 17:56
@Chris: According to your explanation if a protein is present in a complex then it must have some function. –  user677101 Jul 13 at 17:57

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