Protein-chemical characterization of NF-H, the largest mammalian neurofilament component; intermediate filament-type sequences followed by a unique carboxy-terminal extension.

NF-H has the highest mol. wt. of the three mammalian neurofilament components (NF-L, NF-M, NF-H). In spite of its unusually large mol. wt., estimated to be 200 K by gel electrophoresis, NF-H contains sequences which identify it as an integral intermediate filament (IF) protein in its amino-terminal region. We have isolated and partially characterized a basic, non-alpha-helical segment located at the amino-terminal end with properties similar to headpieces of other non-epithelial IF proteins. The highly alpha-helical 40-K fragment excised by chymotrypsin is now identified by the amino acid sequence of a 17-K fragment. This sequence can be unambiguously aligned with the rod region of other IF proteins and covers about half of the presumptive coiled-coil arrays. NF-H and NF-M show 45% sequence identity in this region. The extra mass of NF-H in comparison with most other IF proteins arises from a carboxy-terminal extension thought to be responsible for inter-neurofilament cross-bridges in axons. This autonomous domain has a unique amino acid composition characterized by a high content of proline, alanine and particularly of lysine and glutamic acid. The NF-H tailpiece extension also carries a large number of serine phosphates, which are not evenly distributed, but are restricted to the amino-terminal part. Having now delineated the intermediate filament-type sequences for all three neurofilament proteins it seems very likely that the three components interact via coiled-coil interactions. They all carry unique carboxy-terminal extensions which increase in length from NF-L to NF-H and seem to extend from the filament wall.