THE IMPORTANCE OF INTRAMOLECULAR ION-PAIRING IN INTERMEDIATE FILAMENTS

Nuclear and cytoskeletal networks of 10-nm intermediate filaments (IFs ) are probably ubiquitous in multicellular eukaryotes. They likely pla y a role in maintaining the mechanical integrity of a cell. With the e xception of the nuclear lamins, IF proteins can form IFs in vitro in t he absence of cofactors or associated proteins. Below we present data suggesting that the large alpha-helical ''rod'' domains of IF proteins are stabilized by large numbers (up to 50) of intrahelical ion pairs formed by residues of opposite charge situated four residues apart. Th ese many ion pairs, sometimes involving up to 30% of the residues with in a coiled-coil IF segment, can potentially contribute as much as 10- 25 kcal/mol (1 kcal = 4.18 kJ) to the stability of a single alpha-heli cal rod. Such stabilization is likely to play a major role in the chem ical and physical stability of IF networks in vitro and in vivo. An in vestigation of other coiled-coil proteins shows that selection for int rahelical ion pairing is not simply a property intrinsic to coiled-coi l proteins. Rather, there is a correlation between the degree to which there is selection for intrahelical ion pairs and the extent to which a coiled-coil protein participates in highly ordered multimolecular i nteractions-e.g., as in IFs and myosin thick filaments. The propensity of putative ion pairs in some IF proteins-e.g., epidermal keratins-su ggests that an underlying structural stability at the level of the mon omer may play an important role in the extraordinary stability of dime rs and higher ordered structures in cytoplasmic IFs.