Protein ubiquitination and neuronal differentiation in chick embryos

Ubiquitin is a small highly conserved intracellular protein which is involved in a number of cellular functions including targeting proteins for degradation. We have studied the distribution of ubiquitin-protein conjugates and two enzymes involved in protein ubiquitination in chick embryos. Using immunocytochemical techniques, we have observed that chick neural crest cells and dorsal root ganglia acquire immunoreactivity in their nuclei and cytoplasm as they mature, both in vivo and in vitro, though they are not immunoreactive at early stages of development. Immunoreactivity for the ubiquitin activating enzyme (E1) and a carboxyl terminal hydrolase for ubiquitin (PGP 9.5) also appears in the nuclei of differentiating neurons at the same time as ubiquitin-protein conjugates. Our results provide evidence that the nuclear accumulation of ubiquitin-protein conjugates is closely associated with maturation of neurons towards a differentiated phenotype.