Antibodies directed against ubiquitin inhibit high affinity [³H]choline uptake in rat cerebral cortical synaptosomes

Sodium-dependent [³H]choline uptake and coupled [³H]acetylcholine synthesis were inhibited in rat cerebral cortical synaptosomes in a dose- (1-10 μg/ml) and time-dependent manner by affinity-purified antibodies directed against ubiquitin (anti-Ub). Neither sodium-independent [³H]choline uptake nor [³H]acetylcholine release was affected by up to 10 μg/ml anti-Ub, indicating that the cholinergic terminals were not depolarized by the anti-Ub. Binding of anti-Ub to synaptosomes, as measured with ¹²⁵I-protein A, was saturable and occurred over the same concentration range (1-10 μg/ml) at which uptake inhibition was observed. Although preimmune IgG bound to the synaptosome preparation to a greater extent and was apparently not readily saturable, this fortuitous binding was without effect on high affinity choline uptake and conversion to acetylcholine. The results suggest the presence of a ubiquitin-protein conjugate on the synaptosomal surface and a functional relationship between this protein conjugate and the sodium-dependent choline transport system.