Sterol Carrier Protein-2 Functions in Phosphatidylinositol Transfer and Signaling

• Published in: Biochemistry (Easton) 2003-03, Vol.42 (11), p.3189-3202
• Main Authors: Schroeder, Friedhelm, Zhou, Minglong, Swaggerty, Christina L, Atshaves, Barbara P, Petrescu, Anca D, Storey, Stephen M, Martin, Gregory G, Huang, Huan, Helmkamp, George M, Ball, Judith M
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Blotting, Western; Carrier Proteins - metabolism; Carrier Proteins - physiology; Humans; Inositol 1,4,5-Trisphosphate - metabolism; Intracellular Membranes - metabolism; Liver Neoplasms, Experimental - metabolism; Membrane Proteins - metabolism; Mice; Microsomes - metabolism; Molecular Sequence Data; Phosphatidylinositols - metabolism; Phospholipid Transfer Proteins; Recombinant Proteins - metabolism; Signal Transduction; Subcellular Fractions - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi026904+

Over 20 years ago, it was reported that liver cytosol contains at least two distinct proteins that transfer phosphatidylinositol in vitro, phosphatidylinositol transfer protein (PITP) and a pH 5.1 supernatant fraction containing sterol carrier protein-2 (SCP-2). In contrast to PITP, there has been minimal progress on the structural and functional significance of SCP-2 in phosphatidylinositol transport. As shown herein, highly purified, recombinant SCP-2 stimulated up to 13-fold the rapid (s) transfer of radiolabeled phosphatidylinositol (PI) from microsomal donor membranes to highly curved acceptor membranes. SCP-2 bound to microsomes in vitro and overexpression of SCP-2 in transfected L-cells resulted in the following:  (i) redistribution of phosphatidylinositols from intracellular membranes (mitochondria and microsomes) to the plasma membrane; (ii) enhancement of insulin-mediated inositol-triphosphate production; and (iii) 5.5-fold down regulation of PITP. Like PITP, SCP-2 binds two ligands required for vesicle budding from the Golgi, PI, and fatty acyl CoA. Double immunolabeling confocal microscopy showed SCP-2 significantly colocalized with caveolin-1 in the cytoplasm (punctate) and plasma membrane of SCP-2 overexpressing hepatoma cells (72%), HT-29 cells (58%), and SCP-2 overexpressing L-cells (37%). Taken together, these data show for the first time that SCP-2 plays a hitherto unrecognized role in intracellular phosphatidylinositol transfer, distribution, and signaling.