Protein anisotropy turns solubility on its head.

I n the effort to understand living cells, knowledge about molecular details competes with knowledge of collective properties of large systems of molecules. On the one hand, molecular structures and interactions strongly suggest how the cell’s molecular machines work and give rise to rich metabolic diagrams. On the other hand, subtle collective properties such as the entropy and free energy represent driving forces for chemical reactions, self-assembly, and phase transitions within cells. In a recent issue of PNAS, McManus et al. (1) report a dramatic advance in understanding the differing consequences of protein structure for crystallization and solution clouding. Their work straddles the structural and collective viewpoints and has implications for several current scientific efforts. Just as clouds in the sky reflect attractions between water molecules, forces between proteins lead to phase transitions of clouding and crystallization in solutions. Indeed, analogs of both liquid and ice clouds occur in protein solutions in cataract, the leading cause of blindness (2, 3). However, the complicated surfaces of proteins make for a richer repertoire of transitions than occur for small molecules, and small changes in a protein surface can easily tilt the balance from one transition to another. The dew point of air has an analog in solution called the cloud point. As temperature or other conditions reach the cloud point, droplets of dense or dilute liquid form spontaneously and scatter light, as in clouds. This liquid–liquid coexistence occurs in the oil and water of salad dressing, in molten liquid metal alloys and rocks, in protein solutions (2, 4), in membrane lipids (5), and even in supercooled pure water. It has been extensively analyzed, starting with Van der Waals before 1900 (6). Protein phase transitions, including the liquid–liquid transition and many others, are central to cataract disease (2), sickle-cell disease (7), as well as Alzheimer’s disease and numerous other neurodegenerative diseases (8) and are important in crystal growth (9), cell physiology (10), and industry (11). The work of McManus et al. (1), in the laboratory of George B. Benedek in the Department of Physics at the Massachusetts Institute of Technology, grew out of findings that several single-site mutations alter phase boundaries so as to cause cataract (3, 12–14). To understand the relevant driving forces is challenging because the transitions are sensitive to weak, noncovalent interactions and are affected by the crowded, multicomponent cytoplasm (15–17). Imagine turning a dial to change some water molecules so as to make them freeze when you raised their temperature above the boiling point. Outlandish as that may be, an individual amino acid change does just the analog for the human eye lens protein Dcrystallin (HGD). McManus et al. (1) changed proline 23 to valine, which turned the new P23V protein’s crystallization boundary upside down, so that it crystallized upon raising the temperature instead of upon lowering the temperature (1, 14) (Fig. 1 Upper Left). However, they also found the cloud point boundary for liquid–liquid coexistence to be essentially unchanged (Fig. 1) and set out to discover why. To understand their approach, it is important to recognize that, just as temperature differences drive heat flow, chemical potentials are the collective properties that drive flows of molecules in phase changes, diffusion, and chemi-