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THE PLANT CELL, Vol 7, Issue 12 2175-2185, Copyright © 1995 by American Society of Plant Biologists
Plant Inositol Monophosphatase Is a Lithium-Sensitive Enzyme Encoded by a Multigene Family
G. E. Gillaspy, J. S. Keddie, K. Oda and W. Gruissem
Department of Plant Biology, 111 Koshland Hall, University of California-Berkeley, Berkeley, California 94720-3102
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 LiCI. 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.
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