A Complex Mode of Glucose Signaling

Abstract

In Arabidopsis , hexokinase 1 (HXK1) acts as a glucose sensor to regulate gene expression and plant growth, a role distinct from its function in glycolysis; however, the mechanisms whereby HXK1 mediates glucose signaling have been unclear. After showing that a portion of Arabidopsis HXK was present in the nucleus, Cho et al . used proteomic and binary-interaction screens to identify two proteins--vacuolar H + -ATPase B1 (VHA-B1) and the 19 S regulatory particle of proteasome subunit (RPT5B)--as nuclear-specific HXK1-interacting partners that formed a complex with HXK1. Genetic analysis revealed that vha-B1 and rpt5b loss-of-function mutants resembled the HXK1 gin2 ( glucose-insensitive2 ) mutant: All three mutants were insensitive to repression of cotyledon expansion, chlorophyll accumulation, and leaf and root development in response to high-glucose conditions and showed growth retardation compared with wild-type plants under low-light, low-nutrient conditions. The gin2 -like phenotypes of the vha-B1 and rpt5b mutants did not depend on loss of HXK1 expression, HXK1 protein abundance, or reduction in glucose or fructose phosphorylation. Like gin2 , the vha-B1 and rpt5b mutants were insensitive to glucose-mediated repression of chlorophyll a/b binding protein ( CAB ) and carbonic anhydrase ( CAA ) gene expression. Chromatin immunoprecipitation analysis showed that the HXK1 complex bound to the CAB2 promoter; this binding was reduced but not abolished in the vha-B1 and rpt5b mutants. Thus, these three proteins appear to form a nucleus-specific complex that is critical to glucose-mediated regulation of gene transcription. Y.-H. Cho, S.-D. Yoo, J. Sheen, Regulatory functions of nuclear hexokinase1 complex in glucose signaling. Cell 127 , 579-589 (2006). [PubMed]