Properties of phosphatidylinositol 4-kinase I from suspension cultured Catharanthus roseus cells

A phosphatidylinositol (PI) 4-kinase (ATP:phosphatidylinositol phosphotransferase, EC 2.7.1.67) was extracted from plasma membranes, obtained from suspension cultured Catharanthus roseus cells, and partially purified. An apparent molecular weight of about 500 000 was determined by gel filtration in the presence of Triton X-100 and a pI value of 5.8 by isoelectric focusing. Kinetic parameters and the reaction mechanism were studied at low Triton concentration; K-m values for ATP and PI were found as 62 and 35 mu M, respectively. The equilibrium constant for the PI kinase reaction was found to be 110, i.e, on the side of the products. The kinase catalyses a sequential Pi-Pi reaction; isotope exchange patterns were consistent with a random substrate binding and an ordered product release, with PIP leaving the enzyme before ADP. These findings are similar to the mechanism found in the case of PI kinases from baker's yeast.