Evolutionary diversification of the multimeric states of proteins

One of the most striking features of proteins is their common assembly into multimeric structures, usually homomers with even numbers of subunits all derived from the same genetic locus. However, although substantial structural variation for orthologous proteins exists within and among major phylogenetic lineages, in striking contrast to patterns of gene structure and genome organization, there appears to be no correlation between the level of protein structural complexity and organismal complexity. In addition, there is no evidence that protein architectural differences are driven by lineage-specific differences in selective pressures. Here, it is suggested that variation in the multimeric states of proteins can readily arise from stochastic transitions resulting from the joint processes of mutation and random genetic drift, even in the face of constant directional selection for one particular protein architecture across all lineages. Under the proposed hypothesis, on a long evolutionary timescale, the numbers of transitions from monomers to dimers should approximate the numbers in the opposite direction and similarly for transitions between higher-order structures.