Protein Degradation in Cultured Cells

Rat fibroblasts contain proteins of long and short half-lives, and these two populations can be labeled selectively. Fresh medium contains a factor that inhibits the breakdown of those proteins with long half-lives but has no effect on the rate of breakdown of proteins of short half-lives. Fluoride inhibits the breakdown of both classes of proteins to about the same extent. This effect is immediately reversible, in contrast to the slow reversal of fluoride inhibition of protein synthesis. Iodoacetate also inhibits the breakdown of both classes of proteins to the same extent. The degree of inhibition of protein breakdown by fluoride and iodoacetate does not parallel the effects of these substances on the cellular ATP level. Fluoride causes relatively more inhibition of protein breakdown and relatively less reduction in ATP.