The antigen recognized by the E/C8-monoclonal antibody is expressed in various avian embryonic cell types known also to express neurofilament (NF) immunoreactivity. To determine whether the E/C8-antigen corresponds to any of the known NF components, we compared their subcellular locations, immunocross-reactivities, and electrophoretic behaviors. We found that the E/C8-antibody binds to NF bundles in electron microscope preparations of neurons, but does not correspond to any of the known NF proteins by immunological or electrophoretic criteria. Immunoadsorption with the monoclonal antibody resulted in co-purification of a 73,000-D protein with one of the known NF proteins in homogenates from 20-d embryonic chick brains, but with vimentin intermediate filament protein in similarly prepared homogenates from 4-d embryonic chicks. We suggest that the E/C8-antigen is an intermediate filament-associated protein that binds to different filament types at different stages of development. We have named it NAPA-73, an acronym for neurofilament-associated protein, avian-specific, 73,000 D, on the basis of its binding specificity in mature neurons.

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