Flexible peptides and cytoplasmic gels

In his 2001 book 'Cells, Gels and the Engines of Life' [1], Gerald Pollack gives a highly entertaining and accessible account of cell biology from the standpoint of polymer chemistry. The cytoplasm is indubitably gel-like, he says, so we must expect it to have similar properties to other non-living gels: cells should swell and shrink depending on ionic conditions and undergo dramatic phase changes associated with sol-to-gel transitions. In the broad sweep of published literature on living cells there is indeed abundant support for this thesis, and Pollack is not the first to advance such views. But probably no one else has made the argument so forcefully, or taken it so far. Too far, perhaps, for when Pollack challenges the role of the phospholipid bilayer as a permeability barrier, or offers new and dramatically simplified explanations for such well-understood phenomena as action potentials, muscle contraction and mitosis, he leaves most professional biologists behind [2]. This is unfortunate, since there is much we do not know about the physical conditions existing in the cytoplasm, and here Pollack says much that is relevant - one should not throw out the baby with the bath water.Cytoplasmic gels are associated in my mind with long flexible polymers, which is why I thought about Pollack's book recently when reading about unstructured regions of proteins. We all learned as students that proteins are made as linear chains of peptide-linked amino acids that fold up into stable, evolutionarily-determined three-dimensional structures - right? In fact, wrong! It now appears that enormous numbers of proteins are significantly unfolded under physiological conditions. Some well-characterized proteins appear to be almost completely unstructured in solution, such as the actin-binding protein thymosin and the nucleoporins involved in nuclear transport [3,4]. But many more are now known to contain significant lengths of polypeptide chain that are 'natively unfolded' under con