Squid low molecular weight neurofilament proteins are a novel class of neurofilament protein. A nuclear lamin-like core and multiple distinct proteins formed by alternative RNA processing.
The primary structure of the major 60-kDa squid low molecular mass neurofilament protein (NF60) and a related 70-kDa neurofilament protein have been determined from cDNA clones isolated from a squid brain cDNA library. Structural analysis suggests that the squid NF60 and NF70 neurofilament genes and proteins are remarkably distinct from vertebrate neuronal intermediate filaments characterized previously. Both proteins are encoded on mRNAs generated by alternative RNA processing of the primary transcript of a single gene. Among the known intermediate filament proteins, NF60 and NF70 neurofilament proteins show highest similarity to an epithelial intermediate filament protein from Helix pomatia, a gastropod mollusk, and are less similar to vertebrate neurofilaments. The length of the alpha-helical rod domain in the NF60 and NF70 proteins was reminiscent of the vertebrate nuclear lamins, 6 heptads longer than is found in all known vertebrate cytoplasmic intermediate filaments, in particular the vertebrate neurofilaments. These distinct structural properties suggest that the vertebrate and invertebrate low molecular weight neurofilaments evolved independently from primordial intermediate filament proteins.