Protein subunit assembly

[jane1011]

Are protein subunits always associated or might it be possible that a certain protein modification is required in order for a (e.g. regulatory) subunit to associate with the other subunits?

[JackBean]

look for example to G-protein coupled receptors. They have three subunits which dissociate after receiving signal
http://en.wikipedia.org/wiki/G_protein-coupled_receptor