Npr2, Yeast Homolog of the Human Tumor Suppressor NPRL2, Is a Target of Grr1 Required for Adaptation to Growth on Diverse Nitrogen Sources

Npr2, a putative "nitrogen permease regulator" and homolog of the human tumor suppressor NPRL2, was found to interact with Grr1, the F-box component of the SCFGrr1 (Skp1-cullin-F-box protein complex containing Grr1) E3 ubiquitin ligase, by mass spectrometry-based multidimensional protein identification technology. Npr2 has two PEST sequences and has been previously identified among ubiquitinated proteins. Like other Grr1 targets, Npr2 is a phosphoprotein. Phosphorylated Npr2 accumulates in grr1 Delta mutants, and Npr2 is stabilized in cells with inactivated proteasomes. Phosphorylation and instability depend upon the type I casein kinases (CK1) Yck1 and Yck2. Overexpression of Npr2 is detrimental to cells and is lethal in grr1 Delta mutants. Npr2 is required for robust growth in defined medium containing ammonium or urea as a nitrogen source but not for growth on rich medium. npr2 Delta mutants also fail to efficiently complete meiosis. Together, these data indicate that Npr2 is a phosphorylation-dependent target of the SCFGrr1 E3 ubiquitin ligase that plays a role in cell growth on some nitrogen sources.