INTRODUCTION
Cells synthesize glucose if deprived of it, and destroy gluconeogenic enzymes upon return to glucose-replete conditions. Gluconeogenesis (de novo synthesis of glucose) is, in effect, a reversal of glycolysis, in which glucose is converted to pyruvate. Some enzymatic steps are shared between gluconeogenesis and glycolysis, but other steps are confined to one of the two pathways. In the yeast Saccharomyces cerevisiae, the gluconeogenesis-specific enzymes are fructose-1,6-bisphosphatase (Fbp1), isocitrate lyase (Icl1), malate dehydrogenase (Mdh2), and phosphoenolpyruvate carboxykinase (Pck1).
We found that Gid4, a subunit of the oligomeric GID ubiquitin ligase, is the recognition component of a proteolytic pathway termed the Pro/N-end rule pathway, which conditionally destroys gluconeogenic enzymes. The N-end rule pathway is a set of proteolytic systems whose unifying feature is their ability to recognize and polyubiquitylate proteins containing N-terminal degradation signals called N-degrons, thereby causing the degradation of these proteins by the proteasome. In eukaryotes, the previously known branches of this system are the Arg/N-end rule pathway and the Ac/N-end rule pathway. The Arg/N-end rule pathway targets specific unacetylated N-terminal residues of cellular proteins, including Asn, Gln, Glu, Asp, Arg, Lys,His, Leu, Phe, Tyr, Trp, Ile, and Met (if Met is followed by a bulky hydrophobic residue). The pathwayās other branch, called the Ac/N-end rule pathway, targets proteins for degradation by recognizing their NĪ±-terminally acetylated (Ntacetylated) residues. About 90% of human proteins are cotranslationally and irreversibly Nt-acetylated. Many, possiblymost, Nt-acetylated proteins bear N-degrons of the Ac/N-end rule pathway.
RATIONALE
We wished to identify the recognition component of themultisubunit GID ubiquitin ligase and also to determine whether GID, which was known to mediate the conditional degradation of gluconeogenic enzymes, might recognize them through their N-terminal Pro residues, and also through Pro at position 2.
RESULTS
The successful strategy involved a version of two-hybrid assay for in vivo protein interactions. Themain discovery identified Gid4, a subunit of the GID ubiquitin ligase, as the recognition component (termed Pro/N-recognin) of the Pro/N-end rule pathway. Gid4 was shown to target the gluconeogenic enzymes Fbp1, Icl1, and Mdh2 (and possibly other yeast proteins as well) through the binding to their N-terminal Pro residues in the presence of cognate adjacent sequence motifs. Pck1, the fourth gluconeogenic enzyme, contains Pro at position 2. Gid4 was also required for the degradation of Pck1 through the ability ofGid4 to target the Pro residue of Pck1 at position 2. The properties of Gid4 discovered so far indicate that its substrate binding groove can recognize either the N-terminal Pro residue or Pro at position 2 in the presence of cognate adjacent sequence motifs. The recognition flexibility of Gid4 suggests that the true diversity of Gid4 substrates is o...