Characterization of type III intermediate filament regulatory protein target epitopes: S100 (alpha and/or beta) binds the N-terminal head domain; annexin II2-p112 binds the rod domain

We have investigated the interaction of S-100 proteins (beta and/or alpha) and annexin II2-p11(2) with glial fibrillary acidic protein (GFAP) and desmin to have further information on the mechanisms whereby S-100 proteins and annexin II2-p11(2) affect assembly/disassembly of GFAP and desmin intermediate filaments (IFs). Analyses were conducted on either native IF subunits, GFAP or desmin rod domain, or headless GFAP or desmin. Our data indicate that: (i) S-100 proteins bind to GFAP and desmin N-terminal head domain; (ii) annexin II2-p11(2) binds to GFAP rod domain; (iii) annexin II2-p11(2) does not interact with desmin nor affects desmin assembly. The present data suggest that the ability of S-100 proteins to inhibit GFAP and desmin assemblies and to promote the disassembly of preformed GFAP and desmin IFs depends on occupation of a site on the N-terminal head domain of these IF subunit. It is known that the N-terminal head domain is critical for the progression from the stage of GFAP and desmin dimers/tetramers to that of large oligomers. On the other hand, the ability of annexin II2-p11(2) to stimulate GFAP assembly under conditions where this latter is normally hampered (e.g., at alkaline pH values) might depend on annexin II2-p11(2)-induced changes in the structure of GFAP rod domain, possibly as a consequence of charge modifications. By contrast, the inability of annexin II2-p11(2) to bind to desmin would depend on desmin resistance to charge modifications.