Protein Turnover in Chlorella fusca var. vacuolata: Detection of a Rapidly Degraded Class of Protein in Growing and Non-growing Cells

Abstract

Chlorella fusca var. vacuolata protein was labelled by incorporation of 14C-labelled leucine or tyrosine. On subsequent incubation with unlabelled amino acid in otherwise nitrogen-free medium, radioactive amino acid accumulated in the medium as a consequence of intracellular protein degradation (turnover). The kinetics of loss of label from protein was first order with two major components. By varying the period of incorporation of 14C-labelled amino acid into protein of growing cells, it was deduced that about 8% of protein was degraded at about 20% h−1 in growing cells. The relative size of the ‘unstable’ fraction decreased at the onset of nitrogen starvation. The remaining cell protein was degraded much more slowly during nitrogen starvation and may have been essentially stable in growing cells.