Dissection of the molecular basis of mycophenolate resistance in Saccharomyces cerevisiae

IMP dehydrogenase (IMPDH) is required for the de novo synthesis of guanine nucleotides. While most invertebrates have one IMPDH gene and humans and mice have two, Saccharomyces cerevisiae contains four, IMD1-IMD4. Although Imd2 is 92% identical to Imd3, it is the only S. cerevisiae IMPDH that is resistant to mycophenolic acid in vitro and is the only one of the four that supports drug-resistant growth. Thus, S. cerevisiae is unique in possessing two classes of IMPDH enzymes with very different drug susceptibilities. The mycophenolate-sensitive growth phenotype has become an important genetic tool in yeast, particularly as an indicator for mutations in the transcription elongation machinery. Here we exploit the distinct drug sensitivity of these two closely related IMPDH genes to identify the naturally occurring determinants of drug-resistant growth. Using chimeric IMD2-IMD3 genes in a strain null for IMD genes, we show that one of the 39 amino acid differences between these enzymes is responsible for much of its drug resistance. The IMP dehydrogenase activity of purified chimeric Imd3 containing the Imd2 residue at position 253 was eight-fold more resistant than native Imd3. The reciprocal change in Imd2 resulted in a 23-fold loss of resistance. Hence, acquisition of a hydroxyl side-chain at 523 is sufficient to confer a drug-resistant phenotype upon this organism. We identified the major determinant of the functional distinction between IMD genes in this yeast and suggest that selective pressure on this species forced divergence of one member of this gene family toward drug resistance.