Structural changes in the trichocyte intermediate filaments accompanying the transition from the reduced to the oxidized form

Earlier studies established that substantial changes take place in the three-dimensional structure of the newly assembled trichocyte keratin intermediate filament (IF) during the oxidation process (Wang, H., Parry, D.A.D., Jones, L.N., Idler, W.W., Marekov, L.N., Steinert, P.M. 2000. In vitro assembly and structure of trichocyte keratin intermediate filaments: A novel role for stabilization by disulfide bonding. J. Cell Biol. 151, 1459–1468). The present contribution describes a re-examination of previous data in which more accurate values for the axial dispositions of the molecules have been obtained to yield the most detailed picture yet available of the structural changes that occur in vivo. In particular, it is shown that in the newly assembled (reduced) IF the crosslinking data are consistent with the detailed (8 + 0) model suggested earlier (Fraser, R.D.B., Parry, D.A.D. 2005. The three-dimensional structure of trichocyte (hard α-) keratin intermediate filaments: Features of the molecular packing deduced from the sites of induced crosslinks. J. Struct. Biol. 151, 171–181), in which eight four-chain protofilaments are arranged on an annular ring. For oxidized IF, however, the existing X-ray data require a periodic imperfection in the surface lattice which is substantial in the case of an (8 + 0) model and hence difficult to explain. In contrast, an alternative (7 + 1) model (Fraser, R.D.B., MacRae, T.P., Parry, D.A.D., Suzuki, E. 1986. Intermediate filaments in α-keratin. Proc. Natl. Acad. Sci. USA 83, 1179–1183) requires only a minor imperfection, and it is suggested that this is associated with the central protofilament. This suggestion is shown to be compatible with both the crosslinking data and a model for the axial distribution of electron density derived from the meridional X-ray pattern. In addition, evidence from an X-ray diffraction study of the follicle (Er Rafik, M., Briki, F., Burghammer, M., Doucet, J. 2006. In vivo formation of the hard α-keratin intermediate filament along a hair follicle: Evidence for structural polymorphism. J. Struct. Biol. 154, 79–88) and electron microscope studies of isolated reduced IF (Watts, N.R., Jones, L.N., Cheng, N., Wall, J.S., Parry, D.A.D., Steven, A.C. 2002. Cryo-electron microscopy of trichocyte (hard α-keratin) intermediate filaments reveals a low-density core. J. Struct. Biol. 137, 109–118) have been combined with earlier X-ray studies to give an estimate of the reduction in diameter that occurs in the IF due to the lateral reorganization of the protofilaments during the oxidation process. It has also been shown that the local coiled-coil geometry in the immediate vicinity of the contributing cysteine residues is necessarily disrupted, a feature consistent with the breadths of the near-equatorial layer lines in the X-ray diffraction pattern that indicate an average coherent length of coiled coil of only about 5 nm.