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‐α‐helical segment located at the amino‐terminal end with properties similar to headpieces of other non‐epithelial IF proteins. The highly α‐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 viacoiled‐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.