Amino acid-dependent Gcn4p stability regulation occurs exclusively in the yeast nucleus

The c-Jun-like transcriptional activator Gcn4p controls biosynthesis of translational precursors in the yeast Saccharomyces cerevisiae. Protein stability is dependent on amino acid limitation and cis signals within Gcn4p which are recognized by cyclin-dependent protein kinases, including Pho85p. The Gcn4p population within unstarved yeast consists of a small relatively stable cytoplasmic fraction and a larger less stable nuclear fraction. Gcn4p contains two nuclear localization signals (NLS) which function independently of the presence or absence of amino acids. Expression of NLS-truncated Gcn4p results in an increased cytoplasmic fraction and an overall stabilization of the protein. The same effect is achieved for the entire Gcn4p in a yrb1 yeast mutant strain impaired in the nuclear import machinery. In the presence of amino acids, controlled destabilization of Gcn4p is triggered by the phosphorylation activity of Pho85p. A pho85A mutation stabilizes Gcn4p without affecting nuclear import. Pho85p is localized within the nucleus in the presence or absence of amino acids. Therefore, there is a strict spatial separation of protein synthesis and degradation of Gcn4p in yeast. Control of protein stabilization which antagonizes Gcn4p function is restricted to the nucleus.