AN AUXILIARY FACTOR-CONTAINING A 240-KDA PROTEIN COMPLEX IS INVOLVED IN APOLIPOPROTEIN-B RNA EDITING

A protein complex involved in apolipoprotein B (apoB) RNA editing, ref erred to as AUX240 (auxiliary factor containing p240), has been identi fied through the production of monoclonal antibodies against in vitro assembled 27S editosomes. The 240-kDa protein antigen of AUX240 coloca lized with editosome complexes on immunoblots of native gels, Immunoad sorbed extracts were impaired in their ability to assemble editosomes beyond early intermediates and in their ability to edit apoB RNA effic iently, Supplementation of adsorbed extract with AUX240 restored both editosome assembly and editing activities, Several proteins, in additi on to p240, ranging in molecular mass from 150 to 45 kDa coimmunopurif y as AUX240 under stringent wash conditions. The activity of the catal ytic subunit of the editosome APOBEC-1 and mooring sequence RNA bindin g proteins of 66 and 44 kDa could not be demonstrated in AUX240. The d ata suggest that p240 and associated proteins constitute an auxiliary factor required for efficient apoB RNA editing, We propose that the ro le of AUX240 may be regulatory and involve mediation or stabilization of interactions between APOBEC-1 subunits and editing site recognition proteins leading the assembly of the rat liver C/U editosome.