Molecular and Cellular Biology, May 2004, p. 4083-4091, Vol. 24, No. 10
0270-7306/04/$08.00+0 DOI: 10.1128/MCB.24.10.4083-4091.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Laboratoire de Génétique Moléculaire, Unité de Recherches en Biologie Moléculaire, Facultés Universitaires Notre Dame de la Paix, B-5000 Namur,1 Laboratoire de Chimie Biologique, Université de Mons-Hainaut, B-7000 Mons, Belgium,3 Institut für Genetik, Martin Luther Universität Halle/Wittenberg, D-06099 Halle (Saale), Germany2
Received 18 July 2003/ Returned for modification 19 November 2003/ Accepted 18 February 2004
Utilization of nonfermentable carbon sources by Kluyveromyces lactis and Saccharomyces cerevisiae requires the Snf1p kinase and the Cat8p transcriptional activator, which binds to carbon source-responsive elements of target genes. We demonstrate that KlSnf1p and KlCat8p from K. lactis interact in a two-hybrid system and that the interaction is stronger with a kinase-dead mutant form of KlSnf1p. Of two putative phosphorylation sites in the KlCat8p sequence, serine 661 was identified as a key residue governing KlCat8p regulation. Serine 661 is located in the middle homology region, a regulatory domain conserved among zinc cluster transcription factors, and is part of an Snf1p consensus phosphorylation site. Single mutations at this site are sufficient to completely change the carbon source regulation of the KlCat8p transactivation activity observed. A serine-to-glutamate mutant form mimicking constitutive phosphorylation results in a nearly constitutively active form of KlCat8p, while a serine-to-alanine mutation has the reverse effect. Furthermore, it is shown that KlCat8p phosphorylation depends on KlSNF1. The Snf1-Cat8 connection is evolutionarily conserved: mutation of corresponding serine 562 of ScCat8p gave similar results in S. cerevisiae. The enhanced capacity of ScCat8S562E to suppress the phenotype caused by snf1 strengthens the hypothesis of direct phosphorylation of Cat8p by Snf1p. Unlike that of S. cerevisiae ScCAT8, KlCAT8 transcription is not carbon source regulated, illustrating the prominent role of posttranscriptional regulation of Cat8p in K. lactis.
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