The
mechanisms by which Gal3/KlGal1p activate Gal4p in response to galactose differ
between S. cerevisae and K. lactis.
Alexander Anders, Lutz Kapp, Karin D. Breunig
Institute of Genetics, University of Halle, Weinbergweg 10, Halle, D-06099,
Germany (breunig@genetik.uni-halle.de)
Gal4p is activated by inactivation of Gal80p inhibition by a process that requires interaction between Gal80p and Gal3p or Gal1p. ScGal80p was shown to be retained by Gal3p in the cytoplasm under inducing conditions supporting a model of regulated ScGal80p compartimentation (Peng & Hopper, 2002). Kluyveromyces lactis apparently uses a different mechanism. KlGal80p shows a nuclear localisation in the absence or presence of KlGal1p. For inactivation of nuclear KlGal80p, entry of KlGal1p into the nucleus had to be postulated. A dynamic mathematical model describing the interactions between the three regulators predicted an accumulation of KlGal1p in the nucleus to assure the observed maintenance of induction even under conditions of KlGal80p overexpression. Indeed, KlGal1p could be shown to be enriched in the nucleus in cells overexpressing KlGal80p. We therefore propose that in K. lactis KlGal80p-KlGalp interaction takes place primarily in the nucleus and differs in the mechanism from that proposed for S. cerevisiae. Differences are also evident from the lack of complementation of Klgal1 by GAL3 in K. lactis, different phenotypes resulting from equivalent amino acid exchanges in KlGal80p/ScGal80p and the fact that Gal4p activation is controlled by phosphorylation of Gal80p in K. lactis but not in S. cerevisiae. The phosphorylation sites reside in the non-conserved linker region of KlGal80p that apparently is involved in KlGal1p interaction. Supported by DFG grant Br 921/4.