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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Expression of 1L-Myoinositol-1-Phosphate Synthase in Organelles¹

Department of Biological Sciences, The University of Alabama, Tuscaloosa, Alabama 35487

We have studied the expression of 1L-myoinositol-1-phosphate synthase (MIPS; EC 5.5.1.4) in developing organs of Phaseolus vulgaris to define genetic controls that spatially regulate inositol phosphate biosynthesis. MIPS, the pivotal biosynthetic enzyme in inositol metabolism, is the only enzyme known to catalyze the conversion of glucose 6-phosphate to inositol phosphate. It is found in unicellular and multicellular eukaryotes and has been isolated as a soluble enzyme from both. Thus, it is widely accepted that inositol phosphate biosynthesis is largely restricted to the cytosol. Here, we report findings that suggest the enzyme is also expressed in membrane-bound organelles. Microscopic and biochemical analyses detected MIPS expression in plasma membranes, plastids, mitochondria, endoplasmic reticula, nuclei, and cell walls of bean. To address mechanisms by which the enzyme could be targeted to or through membranes, MIPS genes were analyzed for sorting signals within primary structures and upstream open reading frames that we discovered through our sequence analyses. Comprehensive computer analyses revealed putative transit peptides that are predicted to target the enzyme to different cellular compartments. Reverse transcriptase PCR experiments suggest that these putative targeting peptides are expressed in bean roots and leaves.

² Present address: Department of Plant Biology, The University of California, Davis, CA 95616.

^* Corresponding author; e-mail majohnso{at}bama.ua.edu; fax 205–3481786.

Received January 19, 2003; returned for revision February 20, 2003; accepted February 20, 2003.

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