The N-end rule pathway is required for import of histidine in yeast lacking the kinesin-like protein Cin8p

The N-end rule pathway is a ubiquitin-dependent proteolytic system whose targets include proteins bearing destabilizing N-terminal residues. We carried out a synthetic lethal screen for Saccharomyces cerevisiae mutants that require the N-end rule pathway for cell viability. A mutant thus identified, termed sln2, could not grow in the absence of Ubr1p, the recognition component of the N-end rule pathway, which was not essential for viability of the parental strain under the same conditions. Further analysis showed that inviability of sln2ubr1Delta cells could be rescued either by the HIS3 gene (which was absent from the parental strain) or by a high concentration of histidine in the medium. This defect in histidine uptake, exhibited by the sln2 mutant in the absence but not in the presence of Ubr1p, was traced to the gene HIP1, which encodes the histidine transporter. HIP1 was underexpressed in sln2 ubr1Delta cells, in comparison to either sln2 UBR1 or SLN2 ubr1Delta cells. Yet another property of the sln2 mutant was its inviability at 37 degrees C, which could not be rescued by either UBR1 or HIS3. This feature of sln2 allowed the cloning of SLN2, which was found to be a gene called CIN8, encoding a kinesin-like protein. Thus, either the N-end rule pathway or Cin8p must be present for the viability-sustaining rate of histidine import in S. cerevisiae auxotrophic for histidine. We consider possible mechanisms of this previously unsuspected link between kinesins, ubiquitin-dependent proteolysis, and the import of histidine.