Synthesis, processing, and localization of human Lon protease.

The synthesis, maturation, and localization of the human homolog of the bacterial ATP-dependent Lon protease have been studied in cultured cells and in a cell-free system. Immunofluorescence microscopy of cells transfected with a recombinant human Lon protease-FLAG epitope chimeric protein confirmed the mitochondrial location of human Lon protease. The primary product of transcription and translation directed by a human LON cDNA in vitro had an apparent mass of approximately 107 kDa, and incubation of the translation product with isolated rat liver mitochondria converted the precursor into the approximately 100-kDa mature form. The latter pelleted with mitochondria and was resistant to trypsin digestion. Maturation of human Lon protease in vitro was dependent on mitochondrial inner membrane potential, as the uncoupling agent 2,4-dinitrophenol (DNP) completely blocked this process. In intact cultured cells treated with DNP, newly synthesized Lon protease also accumulated as the precursor form, and subsequent removal of DNP allowed the precursor to be translocated into mitochondria and cleaved to its mature form. A pulse-chase experiment in the absence of DNP showed that the Lon protease precursor is converted to the mature form with a half-time of < 2 min and that the mature human Lon protease is indefinitely stable in intact cells. Submitochondrial fractionation and immunoblot analysis of rat liver mitochondrial proteins using the polyclonal anti-human Lon protease antiserum indicated that the protease is located exclusively in the matrix. These data demonstrate that the maturation of human Lon protease is characterized by steps similar to those reported for other mitochondrial matrix proteins.