Degradative fate of transplanted proteins

The majority of cell proteins are non-cytosolic and are found in specific extracytosolic cytomorphological sites. Rat liver mitochondria and outer mitochondrial membrane (OMM) vesicles were transplanted homologously into rat hepatocytes and heterologously into rat hepatoma (HTC) cells by polyethylene glycol-mediated organelle--cell or OMM vesicle-cell fusion. The subsequent destructive fate of these non-cytosolic proteins was studied. During culture of hepatocyte monolayers in conditions which give in vivo catabolic rates, the transplanted organelle proteins and monoamine oxidase were degraded at rates similar to in vivo rates, although the transplanted material was not found in the hepatocyte mitochondria. Degradation was preceded by internalization (1-6 h) of the transplanted material and its translocation to a perinuclear, vesicular cytoplasmic position. Prevention of translocation by the disruption of the cytoskeleton inhibited subsequent degradation. In contrast, rat OMM heterologously transplanted into HTC cells was patched, capped and internalized into 'unique' vesicles and degraded 2.5 times faster than in hepatocytes. In both hepatocytes and HTC cells mitochondrial protein degradation was partially susceptible to lysosomotropic agents. The results are discussed in terms of a protein turnover cycle which attempts to coordinate the biochemistry and cell biology of protein synthesis and degradation in eukaryotic cells.