Disassembly of synthetic 10-nm desmin filaments from smooth muscle into protofilaments.

Synthetic 10-nm filaments formed from highly purified turkey gizzard desmin have a helically oriented substructure and disassemble into 2 to 2.5 nm protofilaments and 3.5 to 5 nm subfilaments after treatment with 1 or 2 M urea or with low-ionic-strength buffer (8 mM Tris, pH 8.2). SDS-gels of 10-nm filaments treated with these solutions show that a single 55 000-dalton band is present before and after all treatments and indicate that the newly revealed substructure is not caused by proteolysis. Antibodies to electrophoretically purified desmin react, in immunodiffusion, only with the antigen and decorate both the subfilamentous particles and the synthetic 10-nm filaments. These studies indicate that a synthetic 10-nm desmin filament is a rope-like structure constructed from the 2 to 2.5 nm diameter protofilaments.