The Ability of a Charophyte Alga Hexokinase to Restore Glucose Signaling and Glucose Repression of Gene Expression in a Glucose-Insensitive Arabidopsis Hexokinase Mutant Depends on Its Catalytic Activity.

Mikael Ulfstedt,Guo-Zhen Hu,D Magnus Eklund,Hans Ronne

doi:10.3389/fpls.2018.01887

Mikael Ulfstedt, Guo-Zhen Hu + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2018.01887

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Abstract

Hexokinases is a family of proteins that is found in all eukaryotes. Hexokinases play key roles in the primary carbon metabolism, where they catalyze the phosphorylation of glucose and fructose, but they have also been shown to be involved in glucose signaling in both yeast and plants. We have characterized the Klebsormidium nitens KnHXK1 gene, the only hexokinase-encoding gene in this charophyte alga. The encoded protein, KnHXK1, is a type B plant hexokinase with an N-terminal membrane anchor localizing the protein to the mitochondrial membranes. We found that KnHXK1 expressed in Arabidopsis thaliana can restore the glucose sensing and glucose repression defects of the glucose-insensitive hexokinase mutant gin2-1. Interestingly, both functions require a catalytically active enzyme, since an inactive double mutant was unable to complement gin2-1. These findings differ from previous results on Arabidopsis AtHXK1 and its orthologs in rice, where catalytic and glucose sensing functions could be separated, but are consistent with recent results on the rice cytoplasmic hexokinase OsHXK7. A model with both catalytic and non-catalytic roles for hexokinases in glucose sensing and glucose repression is discussed.

Highlights

For all cells, whether they exist as single cells or as a part of a multicellular organism, it is vital to be able to sense if there are nutrients available that can sustain their metabolism and growth
We found that full length KnHXK1 as well as KnHXK1 lacking the N-terminal membrane anchor could complement the hexokinase-deficient yeast strain for use of both glucose and fructose as a carbon source (Figure 4)
We found that the wild type version of KnHXK1 expressed in the gin2-1 mutant restored glucose repression of these genes, behaving similar to the wild type (Figure 9)

Summary

Introduction

Whether they exist as single cells or as a part of a multicellular organism, it is vital to be able to sense if there are nutrients available that can sustain their metabolism and growth. Sugars are important as carbon and energy sources, but they have regulatory roles in many organisms, controlling metabolism, stress resistance, growth and development (Rolland et al, 2001). One is the glucose repression pathway, which involves the Snf protein kinase, the major hexokinase Hxk and the glucose repressor Mig (Entian, 1980; Ronne, 1995; Hohmann et al, 1999; Johnston, 1999; Rolland et al, 2002). In the presence of glucose, Snf is less active, and Hxk and Mig can move into the nucleus and form a repressing complex with Snf on the promoters of glucose-repressed genes (Ahuatzi et al, 2004, 2007; FernándezGarcía et al, 2012). Hxk masks a serine in Mig that is phosphorylated by Snf, protecting Mig from phosphorylation and nuclear export (Pelaez et al, 2010)

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