AN ISOFORM OF THE PHOSPHATIDYLINOSITOL-TRANSFER PROTEIN TRANSFERS SPHINGOMYELIN AND IS ASSOCIATED WITH THE GOLGI SYSTEM

An isoform of the phosphatidylinositol-transfer protein (PI-TP) was id entified in the cytosol fraction of bovine brain. This protein, design ated PI-TP beta, has an apparent molecular mass of 36 kDa and an isoel ectric point of 5.4. The N-terminal amino acid sequence (21 residues) is 90% similar to that of bovine brain PI-TP, henceforth designated PI -TP alpha. (molecular mass 35 kDa and pI 5.5). As observed for PI-TP a lpha, PI-TP beta has a distinct preference for phosphatidylinositol ov er phosphatidylcholine. addition, it expresses a high transfer activit y towards sphingomyelin. PI-TP alpha lacks this activity completely. B y indirect immunofluorescence we demonstrated that, in Swiss mouse 3T3 fibroblasts, PI-TP beta is preferentially associated with the Golgi s ystem whereas PI-TP alpha is predominantly present in the cytoplasm an d the nucleus. In cytosol-depleted HL60 cells, both PI-TP alpha and PI -TP beta were equally effective at reconstituting guanosine 5'-[gamma- thio]triphosphate-mediated phospholipase C beta activity.