Extensive phosphorylation and axonal transport of triton-soluble neurofilament subunits

The low abundance of soluble neurofilament (NF) subunits in mature axons has suggested that newly synthesized NF proteins rapidly assemble into highly stable polymers and associate with the Triton X-100-insoluble cytoskeleton. Here we present evidence for multiple populations of NFs and NF subunits, distinguished by differential solubility in Triton, within perikarya and axons of neurons in situ and in culture. We further demonstrate, using microinjection of "tagged" NF subunits and by pulse-chase radiolabeling of endogenous NF subunits, that these soluble NF populations represent precursors for incorporation into the axonal cytoskeleton.