Evolutionary diversification of the multimeric states of proteins

Significance Rather than operating as single units, most proteins assemble as multimers, usually with all subunits derived from the same gene. In contrast to patterns of gene structure and genome organization, which typically exhibit substantial increases in complexity from unicellular to multicellular organisms, the structural complexity of orthologous proteins appears roughly constant over the tree of life. The interfaces of multimers also often shift dramatically over evolutionary time. To explain these observations, a model is presented for the stochastic origin of variation in the multimeric states of proteins via the joint processes of mutation, random genetic drift, and constant directional selection.