Proline is an amino acid that is not only required for protein synthesis but can also serve as a nitrogen source. Although proline is the least-preferred nitrogen source for many lab strains of S. cerevisiae, it is the most abundant source of nitrogen in grapes, the natural environment of wild yeast (4). When more optimal sources of nitrogen are unavailable, S. cerevisiae cells degrade proline into glutamate via the proline utilization pathway, shown here (1, 5). In the mitochondria, proline is first converted into delta-1-pyrroline-5-carboxylate (P5C) by the PUT1 gene product, proline oxidase (EC 1.5.99.8). Then, P5C is processed by the delta-1-pyrroline-5-carboxylate dehydrogenase (EC 1.5.1.12) Put2p into glutamate (1).

PUT1 and PUT2 are both nuclear genes that are positively regulated by the transcriptional activator Put3p (5). Although Put3p is bound constitutively to the promoters of PUT1 and PUT2, transcriptional upregulation only occurs in the presence of proline and the absence of a preferred nitrogen source (4, 6). In the absence of Put3p, the transcription factor Gal4p can bind at the Put3p binding site and upregulate PUT2 expression (7). PUT1 and PUT2 are also subject to regulation by nitrogen catabolite repression (NCR) (8, 9), which prevents the utilization of proline as a nitrogen source if better nitrogen compounds such as ammonia, asparagine or glutamine are present. PUT2 downregulation by NCR is mediated by the transcription factors Ure2p and Dal80p (9, 10). Independent of NCR, PUT2 expression is repressed by the global transcription factor Spt10p that acts by binding to a TATA element in the PUT2 promoter (11).

During anaerobic respiration, put2 mutations that remove a mitochondrial targeting sequence can enhance cell growth by improving anaerobic arginine catabolism (12). Conversely, under aerobic conditions put2 mutations lead to cell toxicity due to the accumulation of P5C and reactive oxygen species (13). Proline toxicity has also been observed upon mutation of the Arabidopsis ortholog P5CDH (14). In humans, mutation of the PUT2 ortholog, ALDH4A1 (OMIM), causes the genetic disease type II hyperprolinemia (OMIM) which is characterized by elevated levels of P5C, mental retardation, and convulsions (15).

Last updated: 2005-09-07