UNCOUPLING UBIQUITIN PROTEIN CONJUGATION FROM UBIQUITIN-DEPENDENT PROTEOLYSIS BY USE OF BETA-GAMMA-NONHYDROLYZABLE ATP ANALOGS

Pathways of ubiquitin-dependent protein degradation have in common two requirements for ATP. Ubiquitin activation by the enzyme E1 is accompanied by ATP hydrolysis to yield AMP and PP(i), and during conjugate breakdown, the ubiquitin-dependent protease hydrolyzes ATP to ADP and P(i). We show here that either of two beta,gamma-nonhydrolyzable ATP analogues, 5'-adenylyl imidodiphosphate or 5'-adenylyl methylenediphosphate, can support ubiquitin-protein conjugation. With the ubiquitin-dependent protease, however, neither analogue could substitute for ATP. Thus, the substitution of a beta,gamma-nonhydrolyzable analogue for ATP offers a simple method to uncouple ubiquitin conjugation from proteolysis in crude systems. On the basis of pyrophosphate exchange kinetics, E1 has apparent K(m) and V(max) values that are similar for ATP and the analogues, but substrate inhibition by 5'-adenylyl methylenediphosphate made use of the beta,gamma-imido analogue preferable. In one application, beta,gamma-imido-ATP was used in combination with ubiquitin aldehyde (an inhibitor of ubiquitin-protein isopeptidases) to establish that several unfolded RNase A derivatives are recognized equally as ubiquitination substrates. This result extends an earlier study [Dunten, R. L., & Cohen, R. E. (1989) J. Biol. Chem. 264, 16739-16747] to show that conjugate yields, upon which relative ubiquitination rates were based, were not influenced by differential ubiquitin-dependent proteolysis. In a second application, ATP and beta,gamma-imido-ATP were compared in a pulse-chase experiment to investigate the contributions of ATP-dependent proteolysis and isopeptidase activities to conjugate stability.