Different subunits belonging to the same protein complex often exhibit discordant expression levels and evolutionary properties

Hetero-oligomeric protein complexes are involved in many of the key processes in cells. Given that the subunits of a complex function together, it has often been expected to find that (i) they are expressed at similar levels in cells; (ii) they are simultaneously present or absent in genomes; and that (iii) the effects on fitness of deleting their genes should be similar. Such a coherence is, however, often found to be weak or absent. Multi-functionality of subunits and mechanisms of complex assembly are discussed as possible sources for the lack of coherence.