Ubiquitin is involved in the in vitro insertion of monoamine oxidase B into mitochondrial outer membranes.

Monoamine oxidase B that has been synthesized by a reticulocyte lysate charged with bovine liver RNA will insert in a proteinase K-resistant form into isolated outer membranes from rat liver mitochondria. It appears that ubiquitin, a 76-amino acid polypeptide which is enzymatically conjugated to proteins, may be involved in the insertion process. Depletion of endogenous ubiquitin from the reticulocyte lysate with purified antibodies against this polypeptide inhibits the insertion of monoamine oxidase B, and this inhibition is relieved if ubiquitin is restored. On the other hand, a mutant form of ubiquitin which is unable to conjugate with proteins will not support insertion. Conjugation with ubiquitin is an ATP-dependent process. Not only does enzymatic depletion of ATP from the lysate prevent the insertion of monoamine oxidase, but ubiquitin will not restore insertion unless ATP is also present. These data indicate that the formation of a ubiquitin conjugate is involved in the insertion of newly synthesized monoamine oxidase B into the outer membranes.