Two types of phosphatidylinositol 3-kinase from bovine thymus. Monomer and heterodimer form

Two types of phosphatidylinositol (PI) 3-kinase (PI3K) have been purified 6250-fold (PI3KI) and 1250-fold (PI3KII) from the cytosol fraction of bovine thymus. Purified PI3KI and PI3KII were found to have apparent molecular masses of 110 and 190 kDa, respectively, by gel filtration. On the other hand, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, while the molecular mass of PI3KI was again estimated as 110 kDa, PI3KII showed two bands with apparent molecular masses of 110 and 85 kDa, suggesting a heterodimer form. Peptide mapping analysis demonstrated that the 110-kDa protein in PI3KII was the same protein as PI3KI. The specific activity of PI3KI was calculated as 250 nmol/min/mg of protein, while that of PI3KII was 50 nmol/min/mg of protein. The product of PI phosphorylation by PI3KI and PI3KII were confirmed as phosphatidylinositol 3-phosphate by PartiSphere Sax column chromatography. The results show that there are two types of PI 3-kinase in bovine thymus. One exists as a monomer and the other as a heterodimer form. Furthermore, the biochemical properties of these two PI 3-kinases are markedly different. These two types of PI 3-kinase may be regulated differently under physiological conditions.