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.Inositol metabolism is essential for the development of plants, animals, and some microorganisms. Metabolites of inositol (a six carbon cyclitol) such as phosphoinositides and phytate function as potent regulators of signal transduction for a wide variety of hormones, growth factors, and neurotransmitters (York et al., 1999;Loewus and Murthy, 2000; Irvine and Schell, 2001). Inositol phosphate, the immediate precursor of free inositol, is synthesized via the internal cyclization of Glc 6-phosphate. The overall reaction mechanism consists of a tightly coupled oxidation and reduction (Sherman et al., 1969;Loewus and Loewus, 1983). 1l-Myoinositol-1-phosphate synthase (MIPS; EC 5.5.1.4) is the only enzyme known to catalyze this reaction. MIPS is found in diverse organisms, both eukaryotic and prokaryotic, suggesting that the pathway for inositol 1-phosphate biosynthesis from Glc 6-phosphate arose early in the evolution of life (Majumder et al., 1997; Bachhawat and Mande, 2000). The properties and generally accepted catalytic mechanisms of the enzyme are similar in all organisms where such assessment has been undertaken (Loewus and Murthy, 2000). Alignment of MIPS amino acid sequences from diverse organisms including Arabidopsis, bean (Phaseolus vulgaris), Brewer's yeast (Saccharomyces cerevisiae), and Entamoeba histolytica revealed remarkable evolutionary conservation of the primary structure (Majumder et al., 1997). Genome sequencing projects have provided additional evidence for this striking conservation with deduced primary structures from organisms such as Caenorhabditis elegans, fruitfly (Drosophila melanogaster), Leishmania major, and Chlamydomonas reinhardtii.Although the essential...