COMPLEX MECHANISMS FOR C-FOS AND C-JUN DEGRADATION

c-fos and c-jun proto-oncogenes have originally been found in mutated forms in murine and avian oncogenic retroviruses. They both define mul tigenic families of transcription factors. Both c-jun and c-fos protei ns are metabolically unstable. In vivo and in vitro work by various gr oups suggests that multiple proteolytic machineries, including the lys osomes, the proteasome and the ubiquitous calpains, may participate in the destruction of c-fos and c-jun. The relative contribution of each pathway is far from being known and it cannot be excluded that it var ies according to the cell context and/or the physiological conditions. It has been demonstrated that, in certain occurrences, the degradatio n of both c-fos and c-jun by the proteasome in vivo involves the ubiqu itin pathway. However, the possibility that proteasomal degradation ca n also occur in a manner independent of the El enzyme of the ubiquitin cycle remains an open issue.