Formation of 10-nanometer filaments from the 150K-dalton neurofilament protein in vitro.

In the present study we report self-assembly of individual neurofilament (NF) triplet proteins (70K, 150K, and 200K daltons) isolated by anion exchange chromatography from bovine spinal cord. Formation of smooth 10-nm filaments by both NF 150K and NF 70K is shown. Optimal conditions for NK 150K filament formation were incubation in 100 mM MES, 0.2 M NaCl, 1 mM DTT, 0.5 mM EGTA, pH 6.5, at 37 degrees C for 24 hr. Under the same assembly conditions, NF 200K formed 7-nm coiled structures. These thin filaments were similar to those formed by NF 70K and 150K under less than optimal conditions. Our results indicate that NF 150K is an integral part of the filament (self-assembly of NF 70K was previously demonstrated by others). We suggest that the optimal conditions resulting in the formation of a 10-nm 200K homopolymer remain to be determined and that the thin coiled structures formed by all three NF proteins are protofilaments that coalesce to form a double helical 10-nm filament.