A unique neurofilament from Torpedo electric lobe: sequence, expression, and localization analysis.

A set of cDNA clones encoding a protein highly homologous to the mammalian middle-size class of neurofilaments (NF-M) was characterized. The amino acid similarity between the Torpedo and rat NF-M approaches 90% in the amino-terminal "rod-like" domain and is significantly lower in the carboxy-terminal tail. The Torpedo protein contains 13 tandem repeats of a unique six amino acid core, containing a Lys-Ser-Lys putative phosphorylation site. Surprisingly, the 3' untranslated region contains stretches of 80-90% nucleic acid homology with the mammalian, but not with the chicken sequences. This homology is greater than much of the coding region, suggesting that the 3' untranslated region of the message has an important functional role, perhaps governing RNA stability or localization. This Torpedo NF-M mRNA is expressed specifically in the electric lobe and was not detected in other tissues, including brain and spinal cord. A polyclonal antibody generated against a fusion protein synthesized in E. coli detects a 150-kDa protein in the electric lobe and organ, as well as a small amount of material in the brain. Cytochemical studies reveal immunoreactivity in electromotor neuron axons and terminals. Specific expression of neurofilament genes in subsets of central neurons may be important in determining the morphology and functional characteristics of specific neuronal subtypes.