Quinary protein structure and the consequences of crowding in living cells: leaving the test-tube behind.

Although the importance of weak protein-protein interactions has been understood since the 1980s, scant attention has been paid to this "quinary structure". The transient nature of quinary structure facilitates dynamic sub-cellular organization through loose grouping of proteins with multiple binding partners. Despite our growing appreciation of the quinary structure paradigm in cell biology, we do not yet understand how the many forces inside the cell--the excluded volume effect, the "stickiness" of the cytoplasm, and hydrodynamic interactions--perturb the weakest functional protein interactions. We discuss the unresolved problem of how the forces in the cell modulate quinary structure, and to what extent the cell has evolved to exert control over the weakest biomolecular interactions. We conclude by highlighting the new experimental and computational tools coming on-line for in vivo studies, which are a critical next step if we are to understand quinary structure in its native environment.