Lipid transfer proteins

Sterol carrier protein 2: a role in steroid hormone synthesis?

The intracellular movement of cholesterol is an important regulated step in the process of steroidogenesis. However, the molecular mechanisms by which cholesterol is translocated to key organelles, including the mitochondria, remains poorly understood. Lipid transfer proteins may have an important function in this process. One candidate lipid transfer protein is sterol carrier protein 2 (SCP2). This 13.2 kDa protein enhances the movement of cholesterol between vesicles and isolated mitochondria. It also stimulates mitochondrial pregnenolone synthesis. When introduced into intact cells, anti-SCP2 antibodies reduce steroid secretion. Moreover, expression of SCP2 in COS cells engineered to produce progestins increases steroid formation. SCP2 is abundant in steroidogenic glands and the pattern of SCP2 gene expression is consistent with a role for the protein in hormone synthesis: SCP2 transcripts are more prominent in the most steroidogenic compartments of the ovary and tropic hormones that stimulate steroidogenesis increase SCP2 gene expression. Other evidence that suggests that SCP2 plays important roles in cellular function includes a remarkable conservation of primary structure across species. The mechanisms by which SCP2 promotes intracellular sterol movement have not been elucidated. The protein appears to bind sterols and is synthesized with a 20 amino acid N-terminal "pro-" sequence that may serve to target SCP2 to mitochondria. In addition, the C-terminus of SCP2 contains a peroxisome-targeting sequence. SCP2 is derived from a large gene that encodes transcripts that are translated into larger proteins of 30 and 58 kDa. The 58 kDa protein, which has some structural homologies with thiolases, seems to be specifically targeted to peroxisomes whereas SCP2 has a broader subcellular distribution. The significance of the peroxisome association of SCP2 and steroidogenesis has not been disclosed. However, diseases of peroxisome function, including adrenoleukodystrophy and Zellweger syndrome, have notable deficits in steroid and bile acid metabolism, thus linking peroxisomes and steroidogenesis. SCP2 is deficient in fibroblasts of patients with these diseases.

Two yeast peroxisomal proteins crossreact with an antiserum against human sterol carrier protein 2 (SCP-2)

An antibody raised against human sterol carrier protein 2 (SCP-2) crossreacts with two yeast peroxisomal proteins. These proteins have apparent molecular weights of 35 and 58 kDa. Subfractionation of peroxisomes revealed that the 58 kDa species is a soluble matrix protein, whereas the 35 kDa protein is membrane bound. Treatment of isolated peroxisomal membranes with 0.25 M KCl released the 35 kDa crossreactive protein into the soluble supernatant. However, lipid transfer activity could be attributed neither to the 35 kDa nor to the 58 kDa protein.

Amino acid sequence of a non-specific wheat phospholipid transfer protein and its conformation as revealed by infrared and Raman spectroscopy. Role of disulfide bridges and phospholipids in the stabilization of the alpha-helix structure

A wheat non specific phospholipid transfer protein has been isolated from wheat seeds and its amino acid sequence reveals that it is composed of 90 residues for a molecular weight of 9607. From the comparison of its sequence with those of the eight known proteins of the same family, hypotheses on the role of some conserved residues in the transfer activity can be made. The conformation of this protein has been studied by Raman and Fourier transform infrared spectroscopy and this is the first report on the structure of non specific plant phospholipid transfer proteins. As opposed to previous studies on the structure prediction from the amino acid sequence, the results obtained show that plant non specific phospholipid transfer proteins are not almost entirely composed of beta-sheets. Instead, infrared results show that the wheat protein contains 41% alpha-helix and 19% beta-sheet structures, while 40% of the conformation is undefined or composed of turns. Raman spectroscopy shows that three disulfide bridges adopt a gauche-gauche-gauche conformation while the other exhibits a gauche-gauche-trans conformation, and that the two tyrosine residues are hydrogen bonded to water molecules. The cleavage of the disulfide bonds affects significantly the conformation of the protein, the extended confirmation being increased by 15% at the expense of the alpha-helix content. On the other hand, the binding of 1-palmitoyllysophosphatidylcholine to the protein leads to an increase of 8% of the alpha-helix content compared to the free protein. Secondary structure predictions from the amino acid sequence suggest that the binding of a phospholipid stabilizes helicity of the amphipathic helices while the reduction of disulfide bonds would affect the stability of the N-terminal helix. The extended structure located at the C-terminus is not affected. Finally, the wheat phospholipid transfer protein has no effect on the thermotropic behavior of large unilamellar vesicles of dimyristoylphosphatidylcholine while it increases the conformational order of the acyl chains of large unilamellar vesicles of dimyristoylphosphatidylglycerol in the liquid-crystalline state. No major conformational changes of the protein are observed when it is adsorbed to phospholipid vesicles. These results suggest that the helical structure is essential for the transfer activity without excluding a possible role of the C-terminal extended structure on the adsorption to phospholipid vesicles.

Phospholipid transfer proteins: a biological debut

Eukaryotic cells contain a battery of cytosolic proteins that catalyse phospholipid movement in vitro. Current studies are now revealing some surprising aspects of the in vivo function of such proteins, and are also uncovering previously unsuspected relationships between secretory pathway function, intracellular phospholipid transport, phospholipid biosynthesis, and the dynamics of the actin cytoskeleton.