ABSTRACT
Reversible phosphorylation of acidic ribosomal proteins of Saccharomyces cerevisiae is an important mechanism, regulating the number of active ribosomes. The key role in regulation of this process is played by specific, second messenger-independent protein kinases. A new protein-inhibitor regulating activity of PK60S kinase has been purified from yeast extracts and characterised. Peptide mass fingerprinting (PMF) and amino-acid sequence analysis by Post Source Decay (PSD) have identified the inhibitor as a Cu-Zn superoxide dismutase (SOD). Inhibition by SOD is competitive with respect to protein substrates-P proteins and 80S ribosome-with K(i) values of 3.7 microM for P2A protein and 0.6 microM for 80S ribosomes. A close correlation was found between the state of phosphorylation of P proteins in diauxic shift and logarithmic growth yeast cells and activity of SOD. The possible mechanism of regulation of PK60S activity, and participation of SOD protein in regulation of 80S-ribosome activity in stress conditions, is discussed.
