Intermediate filament forming ability of desmin derivatives lacking either the amino-terminal 67 or the carboxy-terminal 27 residues.

Amino acid sequence data and results from limited proteolytic digestion have been used to define the three-domain structure of intermediate filament proteins. A centrally located highly alpha-helical domain of about 310 residues well-conserved in sequence principles and length is flanked by the highly variable sequences of the non-alpha-helical headpiece and tailpiece. A direct involvement in filament formation of one or both terminal domains was previously proposed for desmin since chymotryptic removal of head and tailpiece provided a derivative unable to form filaments. In order to evaluate directly the importance of these regions we have prepared desmin derivatives lacking either the amino-terminal 67 (T-desmin) or carboxy-terminal 27 residues (L-desmin). Whereas the latter derivative is fully polymerization-competent the fragment lacking only the basic and arginine-rich headpiece cannot form filaments on its own and remains in a protofilamentous stage. These structures of T-desmin are not incorporated into filaments when mixed with protofilaments of desmin. If, however, the two proteins are mixed in 7 M-urea subsequent dialysis provides morphologically normal filaments containing T-desmin. The results suggest that at least certain hybrid protofilaments containing less than four headpieces are accepted in the filament. The removal of the 27 carboxy-terminal residues in L-desmin, although not interfering with filament formation, leads to a change in surface since filaments show lateral aggregation at 170 mM but not at 50 mM salt. The results are discussed in relation to current models of intermediate filament structure.