Effect of the growth state on protein turnover in L cells

Abstract

The half-lives of proteins with rapid and slow turnovers were compared in suspension cultures of L cells in growing and non-growing states, by measuring [ 3H]leucine-labeled proteins. When proteins were labeled for a few hours, the half-lives of proteins in the non-growing cells were found to be one-third of those in the growing cells. However, when proteins were labeled for a longer period their half-lives appeared similar in the growing and non-growing states. The protein synthesis decreased and the protein degradation increased as the cell density augmented during cell growth. From the decay curves of proteins labeled for 2 and 20 h in the two growth states it was calculated that the half-lives of proteins with a rapid turnover is 11 h in the non-growing and 29 h in the growing state, and for proteins with a slow turnover it is 40–50 h in both states. The activity of cathepsin D was twice as high in the non-growing as in the growing state, which may explain the difference in protein turnover in the two growth states.