Principles of assembly reveal a periodic table of protein complexes

Structural insights into protein complexes have had a broad impact on our understanding of biological function and evolution. Here we seek a comprehensive understanding of the general principles underlying quaternary structure organisation in protein complexes. To do this, we first examine the fundamental steps by which protein complexes can assemble using experimental and structure-based characterisation of assembly pathways. Most assembly transitions can be classified into three basic types, which can then be used to exhaustively enumerate a large set of possible quaternary structure topologies. These topologies, which include the vast majority of observed protein complex structures, give rise to a natural organisation into a periodic table. Based upon this, we are then able to accurately predict the expected frequencies of quaternary structure topologies, including those not yet observed. Overall, these results have important implications for quaternary structure prediction, modelling and engineering.