Spinach hexokinase I is located in the outer envelope membrane of plastids

AbstractThe subcellular localization of hexokinase activities in plant cells has been a matter of debate for a long time. We have isolated a hexokinase cDNA fragment from glucose-fed spinach leaves using a differential display reverse transcription-PCR approach. The corresponding cDNA was expressed in Escherichia coli and an antiserum, raised against the recombinant protein, was used in subcellular localization studies. The spinach hexokinase could be localized primarily to the outer envelope membrane of chloroplasts where it is inserted via its N-terminal membrane anchor. We suggest that the chloroplast envelope hexokinase is involved in the energization of glucose export from plastids rather than in the sugar-sensing pathway of the plant cell

Identification, Purification, and Molecular Cloning of a Putative Plastidic Glucose Translocator

During photosynthesis, part of the fixed carbon is directed into the synthesis of transitory starch, which serves as an intermediate carbon storage facility in chloroplasts. This transitory starch is mobilized during the night. Increasing evidence indicates that the main route of starch breakdown proceeds by way of hydrolytic enzymes and results in glucose formation. This pathway requires a glucose translocator to mediate the export of glucose from the chloroplasts. We have reexamined the kinetic properties of the plastidic glucose translocator and, using a differential labeling procedure, have identified the glucose translocator as a component of the inner envelope membrane. Peptide sequence information derived from this protein was used to isolate cDNA clones encoding a putative plastidic glucose translocator from spinach, potato, tobacco, Arabidopsis, and maize. We also present the molecular characterization of a candidate for a hexose transporter of the plastid envelope membrane. This transporter, initially characterized more than 20 years ago, is closely related to the mammalian glucose transporter GLUT family and differs from all other plant hexose transporters that have been characterized to date