Cooperation of various subcellular fractions in protein degradation in vitro.

In order to test the possibilities in protein degradation between cell organelles comparatively, [3H]- and [14C]-leucine short-time labelled subcellular fractions from rat liver were incubated with each other at pH 6.9. All fractions tested were able to degrade short-lived proteins from foreign fractions, whereby the lysosomal supernatant fraction showed the highest proteolytic activity, which declines in the sequence: lysosomes--nuclei--mitochondria--cytosol--microsomes. Short-lived cytosolic proteins were especially suited as substrate for neutral proteases from all other fractions, but also microsomal, mitochondrial and nuclear proteins were well degraded by foreign fractions in comparison with the substrate autoproteolysis. Therefore in vivo manyfold cooperations between several organelles in protein catabolism seem to be possible.