Subcellular localization of rice hexokinase (OsHXK) family members in the mesophyll protoplasts of tobacco

Hexokinase (HXK, EC 2.7.1.1) plays an important role in the metabolism and glucose signalling. To examine the characteristics of HXK gene family in rice, the subcellular localizations of ten hexokinases (OsHXK1 - OsHXK10) were determined using OsHXK::GFP fusion proteins in tobacco mesophyll protoplasts. As was previously demonstrated, OsHXK4 was detected in the chloroplast stroma, OsHXK5 and OsHXK6 in the mitochondria, and OsHXK7 and OsHXK10 in the cytoplasm. In the present study, OsHXKs were clearly divided into three types (A, B, C) based on their N-terminal sequences. The new type-C HXKs in plants, OsHXK1, OsHXK7 and OsHXK8, which lack the plastidic transit peptide and the membrane anchor domain, were detected not only in the cytoplasm but also in the nucleus. The type-B HXKs, OsHXK2, OsHXK3, OsHXK9 and OsHXK10, which contained a membrane anchor domain, were distinctly localized in the mitochondria. These results suggest that OsHXKs localized in different cell compartments may be involved in the glucose signalling-related gene expression during growth and development of rice.