Neurophysin biosynthesis: conversion of a putative precursor during axonal transport.

[35S]Cysteine injected adjacent to the supraoptic nucleus of the rat is rapidly incorporated into a 20,000-dalton protein that, in time, is converted to a 12,000-dalton labeled protein, neurophysin. This putative precursor of neurophysin appears to be synthesized in the supraoptic nucleus and transformed to neurophysin and related peptides during axonal transport to the neurohypophysis.