Uptake and degradation of glyceraldehyde-3-phosphate dehydrogenase by rat liver lysosomes.

The molecular mechanisms involved in the degradation of individual cellular proteins are probably unique and characteristic. We have investigated in rat liver the degradation of glyceraldehyde-3-phosphate dehydrogenase, an abundant cytosolic enzyme of the glycolytic pathway. Immunoblot analysis of isolated liver lysosomes from rats treated with lysosomal inhibitors show that this protein is degraded, at least in part, by a lysosomal pathway. This pathway was further investigated by incubating the enzyme with lysosomes in a cell-free system, followed by proteolysis measurements, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of lysosomes, and electron microscopic immunocytochemistry. We postulate that the degradative mechanism of glyceraldehyde-3-phosphate dehydrogenase includes a temperature-dependent lysosomal pathway, different from classical nonspecific macroautophagy. The postulated pathway involves: binding of the enzyme to the lysosomal membrane, entry into the lysosomal matrix, and degradation. This cell-free system, which can also incorporate in vitro synthesized proteins, should allow further advances toward clarifying the complex signals that regulate protein degradation as well as its close interrelationship with protein synthesis.