Membrane binding and insertion of the predicted transmembrane domain of human scramblase 1

Toward a Topology-Based Therapeutic Design of Membrane Proteins: Validation of NaPi2b Topology in Live Ovarian Cancer Cells

NaPi2b is a sodium-dependent phosphate transporter that belongs to the SLC34 family of transporters which is mainly responsible for phosphate homeostasis in humans. Although NaPi2b is widely expressed in normal tissues, its overexpression has been demonstrated in ovarian, lung, and other cancers. A valuable set of antibodies, including L2 (20/3) and MX35, and its humanized versions react strongly with an antigen on the surface of ovarian and other carcinoma cells. Although the topology of NaPi2b was predicted in silico, no direct experimental data are available for the orientation of NaPi2b extracellular domains in cancer cells. The presented results of antibody mapping of untagged NaPi2b in live ovarian carcinoma cells OVCAR-4 provide a platform for current and future epitope-based cancer therapies and serological diagnostics.

Comparative genomic analyses of transport proteins encoded within the red algae Chondrus crispus, Galdieria sulphuraria, and Cyanidioschyzon merolae11

Galdieria sulphuraria and Cyanidioschyzon merolae are thermo-acidophilic unicellular red algal cousins capable of living in volcanic environments, although the former can additionally thrive in the presence of toxic heavy metals. Bioinformatic analyses of transport systems were carried out on their genomes, as well as that of the mesophilic multicellular red alga Chondrus crispus (Irish moss). We identified transport proteins related to the metabolic capabilities, physiological properties, and environmental adaptations of these organisms. Of note is the vast array of transporters encoded in G. sulphuraria capable of importing a variety of carbon sources, particularly sugars and amino acids, while C. merolae and C. crispus have relatively few such proteins. Chondrus crispus may prefer short chain acids to sugars and amino acids. In addition, the number of encoded proteins pertaining to heavy metal ion transport is highest in G. sulphuraria and lowest in C. crispus. All three organisms preferentially utilize secondary carriers over primary active transporters, suggesting that their primary source of energy derives from electron flow rather than substrate-level phosphorylation. Surprisingly, the percentage of inorganic ion transporters encoded in C. merolae more closely resembles that of C. crispus than G. sulphuraria, but only C. crispus appears to signal via voltage-gated cation channels and possess a Na⁺ /K⁺ -ATPase and a Na⁺ exporting pyrophosphatase. The results presented in this report further our understanding of the metabolic potential and toxic compound resistances of these three organisms.

The conformation of human phospholipid scramblase 1, as studied by infrared spectroscopy. Effects of calcium and detergent

Human phospholipid scramblase 1 (SCR) is a membrane protein that catalyzes the transmembrane (flip-flop) motion of phospholipids. It can also exist in a non membrane-bound form in the nucleus, where it modulates several aspects of gene expression. Catalysis of phospholipid flip-flop requires the presence of millimolar Ca²⁺, and occurs in the absence of ATP. Membrane-bound SCR contains a C-terminal α-helical domain embedded in the membrane bilayer. The latter domain can be removed giving rise to a stable truncated mutant SCRΔ that is devoid of scramblase activity. In order to improve our understanding of SCR structure infrared spectra have been recorded of both the native and truncated forms, and the effects of adding Ca²⁺, or removing detergent, or thermally denaturing the protein have been observed. Under all conditions the main structural component of SCR/SCRΔ is a β-sheet. Removing the C-terminal 28 aa residues, which anchor SCR to the membrane, leads to a change in tertiary structure and an increased structural flexibility. The main effect of Ca²⁺ is an increase in the α/β ratio of secondary structure components, with a concomitant increase in the proportion of non-periodic structures. At least in SCRΔ, detergent (Zwittergent 3-12) decreases the structural flexibility, an effect somewhat opposite to that of increasing temperature. Thermal denaturation is affected by Ca²⁺, detergent, and by the presence or absence of the C-terminal domain, each of them influencing in different ways the denaturation pattern.

Phospholipid Scramblase 1 Modulates FcR-Mediated Phagocytosis in Differentiated Macrophages

Phospholipid Scramblase 1 (PLSCR1) was initially characterized as a type II transmembrane protein involved in bilayer movements of phospholipids across the plasma membrane leading to the cell surface exposure of phosphatidylserine, but other cellular functions have been ascribed to this protein in signaling processes and in the nucleus. In the present study, expression and functions of PLSCR1 were explored in specialized phagocytic cells of the monocyte/macrophage lineage. The expression of PLSCR1 was found to be markedly increased in monocyte-derived macrophages compared to undifferentiated primary monocytes. Surprisingly, this 3-fold increase in PLSCR1 expression correlated with an apparent modification in the membrane topology of the protein at the cell surface of differentiated macrophages. While depletion of PLSCR1 in the monocytic THP-1 cell-line with specific shRNA did not inhibit the constitutive cell surface exposure of phosphatidylserine observed in differentiated macrophages, a net increase in the FcR-mediated phagocytic activity was measured in PLSCR1-depleted THP-1 cells and in bone marrow-derived macrophages from PLSCR1 knock-out mice. Reciprocally, phagocytosis was down-regulated in cells overexpressing PLSCR1. Since endogenous PLSCR1 was recruited both in phagocytic cups and in phagosomes, our results reveal a specific role for induced PLSCR1 expression in the modulation of the phagocytic process in differentiated macrophages.

Tubby-like protein superfamily member PLSCR3 functions as a negative regulator of adipogenesis in mouse 3T3-L1 preadipocytes by suppressing induction of late differentiation stage transcription factors

Decrease in intracellular amount of phospholipid scramblase 3 (PLSCR3) is accompanied by enhanced unconventional secretion during differentiation of mouse preadipocytic 3T3-L1 cells. Forced overexpression of PLSCR3 in 3T3-L1 cells inhibited adipogenesis by suppressing induction of late stage pro-adipogenic transcription factors.

PLSCR3 (phospholipid scramblase 3, Scr3) belongs to the superfamily of membrane-associated transcription regulators named Tubby-like proteins (TULPs). Physiological phospholipid scrambling activities of PLSCRs in vivo have been skeptically argued, and knowledge of the biological functions of Scr3 is limited. We investigated the expression of Scr3 during differentiation of mouse 3T3-L1 preadipocytes by Western blotting (WB) and by reverse-transcription and real-time quantitative PCR (RT-qPCR). The Scr3 protein decreased during 3T3-L1 differentiation accompanied by a reduction in the mRNA level, and there was a significant increase in the amount of Scr3 protein secreted into the culture medium in the form of extracellular microvesicles (exosomes). On the other hand, Scr3 expression did not significantly decrease, and the secretion of Scr3 in 3T3 Swiss-albino fibroblasts (a parental cell-line of 3T3-L1) was not increased by differentiation treatment. Overexpression of human Scr3 during 3T3-L1 differentiation suppressed triacylglycerol accumulation and inhibited induction of the mRNAs of late stage pro-adipogenic transcription factors [CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ)] and X-box-binding protein 1 (XBP1). Expression of early stage pro-adipogenic transcription factors (C/EBPβ and C/EBPδ) was not significantly affected. These results suggest that Scr3 functions as a negative regulator of adipogenesis in 3T3-L1 cells at a specific differentiation stage and that decrease in the intracellular amount of Scr3 protein caused by reduction in Scr3 mRNA expression and enhanced secretion of Scr3 protein appears to be important for appropriate adipocyte differentiation.

A cholesterol recognition motif in human phospholipid scramblase 1

Human phospholipid scramblase 1 (SCR) catalyzes phospholipid transmembrane (flip-flop) motion. This protein is assumed to bind the membrane hydrophobic core through a transmembrane domain (TMD) as well as via covalently bound palmitoyl residues. Here, we explore the possible interaction of the SCR TMD with cholesterol by using a variety of experimental and computational biophysical approaches. Our findings indicate that SCR contains an amino acid segment at the C-terminal region that shows a remarkable affinity for cholesterol, although it lacks the CRAC sequence. Other 3-OH sterols, but not steroids lacking the 3-OH group, also bind this region of the protein. The newly identified cholesterol-binding region is located partly at the C-terminal portion of the TMD and partly in the first amino acid residues in the SCR C-terminal extracellular coil. This finding could be related to the previously described affinity of SCR for cholesterol-rich domains in membranes.

Membrane binding of human phospholipid scramblase 1 cytoplasmic domain

Human phospholipid scramblase 1 (SCR) consists of a large cytoplasmic domain and a small presumed transmembrane domain near the C-terminal end of the protein. Previous studies with the SCRΔ mutant lacking the C-terminal portion (last 28 aa) revealed the importance of this C-terminal moiety for protein function and calcium-binding affinity. The present contribution is intended to elucidate the effect of the transmembrane domain suppression on SCRΔ binding to model membranes (lipid monolayers and bilayers) and on SCRΔ reconstitution in proteoliposomes. In all cases the protein cytoplasmic domain showed a great affinity for lipid membranes, and behaved in most aspects as an intrinsic membrane protein. Assays have been performed in the presence of phosphatidylserine, presumably important for the SCR cytoplasmic domain to be electrostatically anchored to the plasma membrane inner surface. The fusion protein maltose binding protein-SCR has also been studied as an intermediate case of a molecule that can insert into the bilayer hydrophobic core, yet it is stable in detergent-free buffers. Although the intracellular location of SCR has been the object of debate, the present data support the view of SCR as an integral membrane protein, in which not only the transmembrane domain but also the cytoplasmic moiety play a role in membrane docking of the protein.

The C-terminal transmembrane domain of human phospholipid scramblase 1 is essential for the protein flip-flop activity and Ca²⁺-binding

Human phospholipid scramblase 1 (SCR) is a 318 amino acid protein that was originally described as catalyzing phospholipid transbilayer (flip-flop) motion in plasma membranes in a Ca²⁺-dependent, ATP-independent way. Further studies have suggested an intranuclear role for this protein in addition. A putative transmembrane domain located at the C terminus (aa 291-309) has been related to the flip-flop catalysis. In order to clarify the role of the C-terminal region of SCR, a mutant was produced (SCRΔ) in which the last 28 amino acid residues were lacking, including the α-helix. SCRΔ had lost the scramblase activity and its affinity for Ca²⁺ was decreased by one order of magnitude. Fluorescence and IR spectroscopic studies revealed that the C-terminal region of SCR was essential for the proper folding of the protein. Moreover, it was found that Ca²⁺ exerted an overall destabilizing effect on SCR, which might facilitate its binding to membranes.