Martinec J., Feltl T., Scanlon C.H., Lumsden P.J., Machackova I.: Localization of high affinity binding site for D - myo inositol 1,4,5-trisphosphate from Chenopodium rubrum. Plant Physiology 2000, 124, 475-483.

It is now generally accepted that a phosphoinositide cycle is involved in the transduction of a variety of signals in plant cells. In animal cells, the binding of D-myo-inositol 1,4,5-trisphosphate (InsP₃) to a receptor located on the endoplasmic reticulum (ER) triggers an efflux of calcium release from the ER. Sites that bind InsP₃ with high affinity and specificity have also been described in plant cells, but their precise intracellular locations have not been conclusively identified. In contrast to animal cells, it has been suggested that in plants the vacuole is the major intracellular store of calcium involved in signal induced calcium release. The aim of this work was to determine the intracellular localization of InsP₃-binding sites obtained from 3-week-old Chenopodium rubrum leaves. Microsomal membranes were fractionated by sucrose density gradient centrifugation in the presence and absence of Mg²⁺ and alternatively by free-flow electrophoresis. An ER-enriched fraction was also prepared. The following enzymes were employed as specific membrane markers: antimycin A-insensitive NADH-cytochrome c reductase for ER, cytochrome c oxidase for mitochondrial membrane, pyrophosphatase for tonoplast, and 1,3-β-glucansynthase for plasma membrane. In all membrane separations, InsP₃-binding sites were concentrated in the fractions that were enriched with ER membranes. These data clearly demonstrate that the previously characterized InsP₃-binding site from C. rubrum is localized on the ER. This finding supports previous suggestions of an alternative non-vacuolar InsP₃-sensitive calcium store in plant cells.