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 identified in the cytosol fraction of bovine brain. This protein, designated PI-TP beta, has an apparent molecular mass of 36 kDa and an isoelectric 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 alpha, PI-TP beta has a distinct preference for phosphatidylinositol over phosphatidylcholine. In addition, it expresses a high transfer activity towards sphingomyelin. PI-TP alpha lacks this activity completely. By indirect immunofluorescence we demonstrated that, in Swiss mouse 3T3 fibroblasts, PI-TP beta is preferentially associated with the Golgi system whereas PI-TP alpha is predominantly present in the cytoplasm and 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.