Identification of myo-Inositol-3-phosphate Synthase Isoforms: CHARACTERIZATION, EXPRESSION, AND PUTATIVE ROLE OF A 16-kDa γc ISOFORM

myo-Inositol is an important constituent of membrane phospholipids and is a precursor for the phosphoinositide signaling pathway. It is synthesized from glucose 6-phosphate by myo-inositol-3-phosphate synthase (IP synthase), a homotrimer composed of a 68-kDa polypeptide in most mammalian tissues. It...

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Published in:The Journal of biological chemistry 2009-04, Vol.284 (14), p.9443-9457
Main Authors: Seelan, Ratnam S, Lakshmanan, Jaganathan, Casanova, Manuel F, Parthasarathy, Ranga N
Format: Article
Language:English
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Summary:myo-Inositol is an important constituent of membrane phospholipids and is a precursor for the phosphoinositide signaling pathway. It is synthesized from glucose 6-phosphate by myo-inositol-3-phosphate synthase (IP synthase), a homotrimer composed of a 68-kDa polypeptide in most mammalian tissues. It is a putative target for mood-stabilizing drugs such as lithium and valproate. Here, we show that the rat gene (Isyna1) encoding this enzyme generates a number of alternatively spliced transcripts in addition to the fully spliced form that encodes the 68-kDa subunit (the α isoform). Specifically, we identify a small 16-kDa subunit (the γc isoform) derived by an intron retention mechanism and provide evidence for its existence in rat tissues. The γc isoform is highly conserved in mammals, but it lacks the catalytic domain while retaining the NAD⁺ binding domain. Both α and γc isoforms are predominantly expressed in many rat tissues and display apparent stoichiometry in purified enzyme preparations. An IP synthase polyclonal antibody not only detects the α and γc isoforms but also several other isoforms in pancreas, intestine, and testis suggesting that the holoenzyme is composed of unique subunits in various tissues. Interestingly, the α isoform is not expressed in the intestine. IP synthase activity assays using purified α and γc isoforms indicate that the latter negatively modulates α isoform activity, possibly by competing for NAD⁺ molecules. Our findings have important ramifications for understanding the mood stabilization process and suggest that inositol biosynthesis is a highly regulated and dynamic process.
ISSN:0021-9258
1083-351X
DOI:10.1074/jbc.M900206200