Localization of phosphatidylinositol 4-kinase isoenzymes in rat liver plasma membrane domains.

The presence of different isoenzymes of phosphatidylinositol 4-kinase in isolated rat liver plasma membranes and their further distribution in plasma membrane domains was examined. Both wortmannin-sensitive and -insensitive PtdIns 4-kinase activities were detected in highly purified plasma membranes obtained by aqueous two-phase affinity partitioning. The wortmannin-sensitive enzyme was identified as the 230 kDa isoform by Western blotting, whereas the 92 kDa isoform was not detected in plasma membranes. The apparent molecular weights of these isoforms were 205 and 105 kDa on SDS polyacrylamide gel electrophoresis, but approximately 500 and 230 kDa respectively on gel filtration, suggesting that both enzymes either are dimers or composed of heterologous subunits. Approximately 25% of the total 230 kDa isoenzyme present in liver, and only ca 5% of the wortmannin-insensitive one, was associated with the plasma membrane fraction. Plasma membrane domains were isolated by a combination of sucrose and Nycodenz gradient centrifugations. The 230 kDa isoform was identified in the blood sinusoidal domain, but not in the bile canalicular one, and was also found in lateral plasma membranes. The wortmannin-insensitive isoenzyme was present only in this latter material. The functional implications of this distribution of PtdIns 4-kinase isoenzymes in plasma membrane regions are discussed.