Requirement for ubiquitin conjugation and 26S proteasome activity at an early stage in V(D)J recombination

V(D)J recombination, the process that rearranges gene segments to assemble mature antigen receptor genes, relies on a recombinase comprising the RAG1 and RAG2 proteins. RAG1 is a multi-functional enzyme including DNA binding and cleavage as well as ubiquitin ligase activities, all of which appear to contribute to its role in recombination. Here we demonstrate that components of the ubiquitin conjugation machinery and the 26S proteasome are required for an early step in V(D)J recombination. Inhibitors of the 26S proteasome and ubiquitin activating enzyme (E1) blocked both chromosomal and extra-chromosomal recombination when added 1 h following transfection/induction, but they had no effect when added 16 h later. There was no effect on expression of RAG1, and recombination did not require transit through the cell cycle, confirming that inhibition was not due to an indirect effect on cell cycle arrest or protein expression. Experiments in which RAG1 translation was blocked with cyclohexamide after 16 h of expression indicated that many active recombination complexes were formed within this window, although recombination products continued to accumulate for 48 h. These data suggest that ubiquitin-dependent degradation is an early step in complex assembly or activation, and are consistent with our previous hypothesis that degradation of a negative regulator is required to trigger recombination.