Studies on the biosynthesis of neurofilament proteins.

To determine whether the triplet polypeptides of neurofilaments arise by degradation of precursor, we studied the biosynthesis of neurofilament polypeptides both in vivo and in cell-free systems. Neurofilament-enriched fractions and polyribosomes were prepared from the same rabbit spinal cord homogenates. At 1 h after intracisternal administration of [34S]methionine, radiolabeled neurofilament proteins were detected in spinal cord homogenates as well as in isolated filaments. When polyribosomes from rabbit spinal cord were allowed to incorporate [35S]methionine into protein, triplet polypeptides were among the proteins labeled. Addition of spinal cord polyribosomes to rabbit reticulocyte lysates led to several cycles of translation of the spinal cord mRNA; the three neurofilament polypeptides were among the proteins synthesized in this system. The results demonstrate that the triplet polypeptides of neurofilaments are synthesized as such in the course of individual translational events and do not arise from degradation of P200 or a larger precursor.