Figure 5

A possible evolutionary scenario for incremental duplication of whole complexes. a) Ancestral three-subunit protein complex before duplications. b) Intra-complex duplication, gives rise to paralogous proteins, mutually exclusive as a part of the complex. Two module variants arise, with either A' or A" as one of subunits. c) Another intra-complex duplication makes possible assembly of up to four distinct module variants. d) Paralogs co-evolve to work together, specializing to accept only one of the interacting paralogs. This gives rise to more accentuated module variants, which now can be interpreted as separate complexes with a shared subunit. e) Eventually, all subunits may get duplicated, creating two independent protein complexes.