The polyphosphoinositides,phosphatidylinositol monophosphate and phosphatidylinositol bisphosphate,are present in nuclei isolated from carrot protoplast

Summary Nuclei were isolated from carrot protoplasts and the distribution of [³H]inositol-labeled phospholipids was analyzed by thinlayer chromatography. Phosphatidylinositol (PI), lysophos-phatidylinositol (LPI), phosphatidylinositol monophosphate (PIP), lysophosphatidylinositol monophosphate (LPIP), and phosphatidylinositol bisphosphate (PIP₂) were 55.7%, 12.3%, 5.0%, 11.5%, and 3.6% of the respective [³H]inositol-labeled lipids recovered from the nuclear fraction. While both the plasma membrane and nuclear fraction contained polyphosphoinositides, the distribution of the phosphoinositides and the amount of inositol-labeled lipid were distinct. For example, the nuclear fraction had a higher percentage of LPI and PIP₂ and less PI and LPIP than the plasma membrane fraction. The amount of [³H]inositol-labeled lipid recovered from the nuclear fraction per mg protein was an order of magnitude lower than that recovered from either the plasma membrane of lower phase fraction isolated by aqueous two-phase partitioning, or from whole cells and protoplasts. In addition, when the ratio of the [³H]inositol-labeled lipid was compared to total [¹⁴C]myristate-labeled lipid recovered there was three to ten fold less [³H] relative to [¹⁴C] in the nuclear fraction.These data indicate that while the polyphosphoinositides are a relatively high percentage of the inositol lipid in the nuclear fraction, the inositol lipid was only a small portion of the total lipid in the nuclei. Despite this low concentration of inositol lipid, when [³²P]-ATP was added to the isolated nuclei,³²P-labeled PIP and PIP₂ were synthesized. Thus, the carrot nuclei contained PI and PIP kinase as well as the polyphosphoinositides.Abbreviations PI phosphatidylinositol - LPI lysophosphatidylinositol - PIP phosphatidylinositol monophosphate - LPIP lysophosphatidylinositol monophosphate - PIP₂ phosphatidylinositol bisphosphate - DAG diacylglycerol - IP₃ inositol 1,4,5-trisphosphate