Evidence of heterogeneity of protein-turnover states in cultured cells

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Abstract
Previous studies on [mouse fibroblast] L-cell cultures suggested that degradation of slow-turnover proteins occurs in a distinct cell state (D-state) and that cells randomly enter the D-state with a 1st-order transition constant, rapidly degrade cell protein and return to a quiescent G0-state. The hypothesis that the putative D-state exists as a substate, within A-state (non-replicating) fibroblasts was tested. Rat-embryo fibroblasts were prelabeled with [14C]leucine and [3H]thymidine, chased for 24 h, and then placed in fresh growth medium containing either vinblastine (10 .mu.M) or colchicine (25 .mu.M) for 3 successive 24 h periods. Cells trapped in mitosis were separated from the residual non-replicating cells, and rates of protein synthesis, degradation and net accumulation were measured in both populations. Significant protein degradation occurred only in the non-replicating population, although both populations showed equally high rates of protein synthesis induced by fresh growth medium. Degradation of slow-turnover protein is apparently heterogeneous, occurring only in A-state cells. A model that proposes a separate D-state within G0-phase successfully accounts for these observations and previous reports on this cell line, showing no differences in degradation of the slow turnover protein pool in growth-stimulated and stationary phase fibroblast cultures.