Protein degradation in rat liver. Evidence for populations of protein degradation rates in cellular organelles.

Protein degradation in liver subcellular and submitochondrial fractions from adult rats has been measured by a double isotope technique. Protein subunits have been resolved by two-dimensional polyacrylamide gel electrophoresis. No consistent relationship between subunit size or isoelectric point and degradation rate has been found in any fraction. Frequency analysis of isotope ratios for protein subunits in subcellular and submitochondrial fractions has been carried out. The analyses demonstrate that degradation rates of resolved protein subunits in organelles or organelle subcompartments can be grouped into populations. Resolved protein subunits in subcellular and submitochondrial fractions can be grouped into one, two or three populations. The results suggest limited heterogeneity of protein degradation rates within each subcellular organelle and support the notion that morphological subcompartment rather than molecular property may be the prime determinant of rates of degradation of individual proteins.