Distinct Roles Of Galactose-1P In Galactose-mediated Growth Arrest Of Yeast Deficient In Galactose-1P Uridylyltransferase (GALT) and UDP-galactose 4'-epimerase (GALE)

• Authors: Jane Odhiambo Mumma, Emory University; Juliet S. Chhay, Emory University; Kerry L. Ross, Emory University; Jana S. Eaton, Emory University; Karen A. Newell-Litwa, Emory University; Judith L. Fridovich-Keil, Emory University
• Journal Title: Molecular Genetics and Metabolism
• Volume: Volume 93, Number 2
• Publisher: Elsevier: 12 months | 2008-02, Pages 160-171
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1016/j.ymgme.2007.09.012
• Subjects: Biology, Molecular; Biology, Genetics
• Keywords: galactose; galactosemia; yeast; gal-1P; GALT; GALE; GALK

Abstract

Galactose is metabolized in humans and other species by the three-enzyme Leloir pathway comprised of galactokinase (GALK), galactose 1-P uridylyltransferase (GALT), and UDP galactose 4'-epimerase (GALE). Impairment of GALT or GALE in humans results in the potentially lethal disorder galactosemia, and loss of either enzyme in yeast results in galactose-dependent growth arrest of cultures despite the availability of an alternate carbon source. In contrast, loss of GALK in humans is not life-threatening, and in yeast has no impact on the growth of cultures challenged with galactose. Further, the growth of both GALT-null and GALE-null yeast challenged with galactose is rescued by loss of GALK, thereby implicating the GALK reaction product, gal-1P, for a role in the galactose-sensitivity of both strains. However, the nature of that relationship has remained unclear. Here we have developed and applied a doxycycline-repressible allele of galactokinase to define the quantitative relationship between galactokinase activity, gal-1P accumulation, and growth arrest of galactose-challenged GALT or GALE-deficient yeast. Our results demonstrate a clear threshold relationship between gal-1P accumulation and galactose-mediated growth arrest in both GALT-null and GALE-null yeast, however, the threshold for the two strains is distinct. Further, we tested the galactose-sensitivity of yeast double-null for GALT and GALE, and found that although loss of GALT barely changed accumulation of gal-1P, it significantly lowered the accumulation of UDP-gal, and also dramatically rescued growth of the GALE-null cells. Together, these data suggest that while gal-1P alone may account for the galactose-sensitivity of GALT-null cells, other factors, likely to include UDP-gal accumulation, must contribute to the galactose-sensitivity of GALE-null cells.