Move over Mendeleev, there’s a new periodic table in science. Unlike the original periodic table, which organized the chemical elements, the new periodic table organizes protein complexes, or more precisely, quaternary structure topologies. Though there are other differences between the old and new periodic tables, they share at least one important feature—predictive power.
When Mendeleev introduced his periodic table, he predicted that when new chemical elements were discovered, they would fill his table’s blank spots. Analogous predictions are being ventured by the scientific team that assembled the new periodic table. This team, consisting of scientists from the Wellcome Genome Campus and the University of Cambridge, asserts that its periodic table reveals the regions of quaternary structure space that remain to be populated.
The periodic table of protein complexes not only offers a new way of looking at the enormous variety of structures that proteins can build in nature, it also indicates which structures might be discovered next. Moreover, it could point protein engineers toward entirely novel structures that never occurred in nature, but could be engineered.
The new table appeared December 11 in the journal Science, in an article entitled, “Principles of assembly reveal a periodic table of protein complexes.” The “principles of assembly” referenced in this title amount to three basic assembly types: dimerization, cyclization, and heteromeric subunit addition. In dimerization, one protein complex subunit doubles, and becomes two; in cyclization, protein complex subunits from a ring of three or more; and in heteromeric subunit addition, two different proteins bind to each other.
These steps, repeated in different combinations, gives rise to enormous number of proteins of different kinds. “Evolution has given rise to a huge variety of protein complexes, and it can seem a bit chaotic,” explained Joe Marsh, Ph.D., formerly of the Wellcome Genome Campus and now of the MRC Human Genetics Unit at the University of Edinburgh. “But if you break down the steps proteins take to become complexes, there are some basic rules that can explain almost all of the assemblies people have observed so far.”
The authors of the Science article noted that many protein complexes assemble spontaneously via ordered pathways in vitro, and these pathways have a strong tendency to be evolutionarily conserved. “[There] are strong similarities,” the authors added, “between protein complex assembly and evolutionary pathways, with assembly pathways often being reflective of evolutionary histories, and vice versa. This suggests that it may be useful to consider the types of protein complexes that have evolved from the perspective of what assembly pathways are possible.”
To explore this rationale, the authors examined the fundamental steps by which protein complexes can assemble, using electrospray mass spectrometry experiments, literature-curated assembly data, and a large-scale analysis of protein complex structures. Ultimately, they derived their approach to explaining the observed distribution of known protein complexes in quaternary structure space. This approach, they insist, provides a framework for understanding their evolution.
“In addition, it can contribute considerably to the prediction and modeling of quaternary structures by specifying which topologies are most likely to be adopted by a complex with a given stoichiometry, potentially providing constraints for multi-subunit docking and hybrid methods,” the authors concluded. “Lastly, it could help in the bioengineering of protein complexes by identifying which topologies are most likely to be stable, and thus which types of essential interfaces need to be engineered.”
The rows and columns of the periodic table of the elements, called periods and groups, were originally determined by each element’s atomic mass and chemical properties, later by atomic number and electron configuration. In contrast, the rows and columns of the periodic table of protein complexes correspond to the number of different subunit types and the number of times these subunits are repeated. The new table is not, it should be noted, periodic in the same sense as the periodic table of the elements. It is in principle open-ended.
Although there are no theoretical limitations to quaternary structure topology space in either dimension, the abridged version of the table presented in the Science article can accommodate the vast majority of known structures. Moreover, when the table’s creators compared the large variety of countenanced topologies to observed structures, they found that about 92% of known protein complex structures were compatible with their model.
“Despite its strong predictive power, the basic periodic table model does not account for about 8% of known protein complex structures,” the authors conceded. “More than half of these exceptions arise as a result of quaternary structure assignment errors.
“A benefit of this approach is that it highlights likely quaternary structure misassignments, particularly by identifying nonbijective complexes with even subunit stoichiometry. However, this still leaves about 4% of known structures that are correct but are not compatible with the periodic table.” The authors added that the exceptions to their model are interesting in their own right, and are the subject of ongoing studies.