The conserved H1 domain of the type II keratin 1 chain plays an essential role in the alignment of nearest neighbor molecules in mouse and human keratin 1/keratin 10 intermediate filaments at the two- to four-molecule level of structure.
A number of fundamental questions pertaining to the registration and packing of the constituent coiled-coil molecules in keratin intermediate filaments, and to the regions of the sequences that are responsible for these levels of organization, remain to be elucidated. In this study, small assembly-competent oligomers of mouse and human keratin 1/keratin 10 keratin filaments were cross-linked by the formation of disulfide bonds catalyzed by the copper-phenanthroline reaction. By isolation and characterization of cross-linked peptides, it has been possible to establish two major modes of molecule alignment: an antiparallel arrangement of half-staggered molecules with their 2B segments overlapping and an antiparallel arrangement of molecules in close axial registration. These data confirm earlier models based on theoretical considerations (Crewther, W. G., Dowling, L. M., Steinert, P. M., and Parry, D. A. D. (1983) Int. J. Biol. Macromol. 5, 267-274). Interestingly, these models place the conserved H1 and H2 end domain segments, which flank the ends of the rod domains of the type II keratin 1 chain, in alignment with either the ends of the rod domains and/or with the L2 segment near the center of the rod domains, of the nearest neighbor molecules. Competition experiments with synthetic peptides suggest that the conserved H1 (and possibly H2) subdomain sequences unique to type II keratin chains play pivotal roles in the registration of neighboring molecules in keratin filaments. The data thus afford a molecular explanation for why keratin filaments require a type II chain for assembly in vivo and in vitro.