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