Characterization
of Krh1, a novel signal transduction component of the protein kinase A pathway
in Saccharomyces cerevisiae.
Ruud Gelade,
David Nauwelaers, Johan M. Thevelein
Lab of Molecular Cell Biology, Katholieke Universiteit Leuven, Kasteelpark
Arenb 31, Leuven-Heverlee, 3001, Belgium (ruud.gelade@bio.kuleuven.ac.be)
In baker's yeast the protein kinase A pathway stimulates the metabolic switch to fermentation as soon as a sufficient amount of glucose is present in the medium. Stimulation of adenylate cyclase activity by glucose appears to be mediated by the heterotrimeric G-alpha protein Gpa2 and the coupled receptor-like protein Gpr1. This GPCR system signals through protein kinase A and thereby controls important cellular properties such as growth and stress resistance. Recently the YAL056w gene product was isolated in a two-hybrid screen using the G-alpha protein Gpa2 as bait. Yal056w and its homologue Yor371c contain a seven times repeated kelch motif.. Hence, we renamed the YAL056w gene KRH1 and the YOR371c gene KRH2 for 'Kelch Repeat Homologues 1 and 2'. A krh1 mutant has a high PKA-phenotype. Trehalose and glycogen levels are lowered and trehalase activity is higher in the deletion mutant compared to the wild type. The opposite is true for the Krh1 overexpression mutant. The overall stress-resistance is drastically reduced when Krh1 is deleted. Subsequent deletions of Gpa2 and Gpr1 in these Krh1 mutants were unable to counteract the effects on trehalose and glycogen levels and on stress-resistance. In addition, basal cAMP levels and glucose-induced cAMP increases were only slightly affected in the Krh1 deletion and overexpression mutants. These results indicate a possible role for Krh1 independent and/or downstream of the glucose sensing module Gpr1-Gpa2 and adenylate cyclase