Abstract
The analysis of glucose signaling in the Crabtree-positive eukaryotic model organism Saccharomyces cerevisiae has disclosed a dual role of its hexokinase ScHxk2, which acts as a glycolytic enzyme and key signal transducer adapting central metabolism to glucose availability. In order to identify evolutionarily conserved characteristics of hexokinase structure and function, the cellular response of the Crabtree-negative yeast Kluyveromyces lactis to rag5 null mutation and concomitant deficiency of its unique hexokinase KlHxk1 was analyzed by means of difference gel electrophoresis. In total, 2,851 fluorescent spots containing different protein species were detected in the master gel representing all of the K. lactis proteins that were solubilized from glucose-grown KlHxk1 wild-type and mutant cells. Mass spectrometric peptide analysis identified 45 individual hexokinase-dependent proteins related to carbohydrate, short-chain fatty acid and tricarboxylic acid metabolism as well as to amino acid and protein turnover, but also to general stress response and chromatin remodeling, which occurred as a consequence of KlHxk1 deficiency at a minimum 3-fold enhanced or reduced level in the mutant proteome. In addition, three proteins exhibiting homology to 2-methylcitrate cycle enzymes of S. cerevisiae were detected at increased concentrations, suggesting a stimulation of pyruvate formation from amino acids and/or fatty acids. Experimental validation of the difference gel electrophoresis approach by post-lysis dimethyl labeling largely confirmed the abundance changes detected in the mutant proteome via the former method. Taking into consideration the high proportion of identified hexokinase-dependent proteins exhibiting increased proteomic levels, KlHxk1 is likely to have a repressive function in a multitude of metabolic pathways. The proteomic alterations detected in the mutant classify KlHxk1 as a multifunctional enzyme and support the view of evolutionary conservation of dual-role hexokinases even in organisms that are less specialized than S. cerevisiae in terms of glucose utilization.