Plant inositol monophosphatase is a lithium-sensitive enzyme encoded by a multigene family.

Abstract

myo-Inositol monophosphatase (IMP) is a soluble, Li(+)-sensitive protein that catalyzes the removal of a phosphate from myo-inositol phosphate substrates. IMP is required for de novo inositol synthesis from glucose 6-phosphate and for breakdown of inositol trisphosphate, a second messenger generated by the phosphatidylinositol signaling pathway. We cloned the IMP gene from tomato (LeIMP) and show that the plant enzyme is encoded by a small gene family. Three different LeIMP cDNAs encode distinct but highly conserved IMP enzymes that are catalytically active in vitro. Similar to the single IMP from animals, the activities of all three LeIMPs are inhibited by low concentrations of LiCl. LeIMP mRNA levels are developmentally regulated in seedlings and fruit and in response to light. Immunoblot analysis detected three proteins of distinct molecular masses (30, 29, and 28 kD) in tomato; these correspond to the predicted molecular masses of the LeIMPs encoded by the genes. Immunoreactive proteins in the same size range are also present in several other plants. Immunolocalization studies indicated that many cell types within seedlings accumulate LeIMP proteins. In particular, cells associated with the vasculature express high levels of LeIMP protein; this may indicate a coordinate regulation between phloem transport and synthesis of inositol. The presence of three distinct enzymes in tomato most likely reflects the complexity of inositol utilization in higher plants.

Lithium treatment induces a hypersensitive-like response in tobacco

“…Lithium accumulates in leaves of treated plants, and inhibition of known targets of this cation, such as inositol monophosphatases (Gillaspy et al 1995), Hal2-like nucleotidases (Murguı´a et al 1996;Gil-Mascarell et al 1999), and/or rRNA-processing enzymes (Dichtl et al 1997) could be involved. Inhibition of inositol monophosphatases would affect the inositol cycle and calcium signalling (Nahorski et al 1991;Liang et al 1996), while inhibition of Hal2-like nucleotidases would affect sulphotransferase reactions and rRNA processing (Dichtl et al 1997;Gil-Mascarell et al 1999).…”

“…Although inhibition of glycogen synthase kinase-3b has been proposed as a possible mechanism for the effect of lithium on development (Klein and Melton 1996), the most widely accepted model for lithium action is the ''inositol depletion hypothesis'' (Berridge 1993), which is based on the inhibition of inositol monophosphatases by Li + , leading to depletion of cellular inositol and a rundown of the inositol cycle and calcium signalling. Plant inositol monophosphatases are Li + -sensitive (Gillaspy et al 1995) and the effects of Li + on plants have also been interpreted in terms of inhibition of the inositol cycle and calcium signalling Tupy 1995a, 1995b;Liang et al 1996).…”

The Rhizobium leguminosarum bv. trifolii pssB gene product is an inositol monophosphatase that influences exopolysaccharide synthesis

“…The gene is located upstream of pssA and separated by 780 bp intergenic region, but is not linked to other known pss genes. pssB encodes a protein with a deduced M r of 28.36 kDa (Janczarek et al 1999), and it showed a moderate sequence identity to CysQ (41.2% identity) and SuhB (26.2% identity) from E. coli (Matsuhisa et al 1995;Neuwald et al 1992) and about 25% identity to the family of plant and mammalian IMPases (Diehl et al 1990;Gillaspy et al 1995). These similarities led us to test the IMPase activity of PssB protein.…”

Inositol 1,4,5 trisphosphate is inactivated by a 5-phosphatase in stamen hair cells of Tradescantia

“…Moreover, the products, Ins(4,5)P 2 and Ins(1,4)P 2 , have been shown to be further hydrolyzed to monophosphates, then to inositol (Joseph et al 1989;Memon et al 1989;Drobak et al 1991;Martinoia et al 1993;Gillaspy et al 1995). Yet other studies provide support in plants for 6-kinase activity in which Ins(1,4,5)P 3 is converted to Ins(1,4,5,6)P 4 (Chattaway et al 1992).…”