Calcium-dependent phospholipid scrambling by TMEM16F.

In all animal cells, phospholipids are asymmetrically distributed between the outer and inner leaflets of the plasma membrane. This asymmetrical phospholipid distribution is disrupted in various biological systems. For example, when blood platelets are activated, they expose phosphatidylserine (PtdSer) to trigger the clotting system. The PtdSer exposure is believed to be mediated by Ca(2+)-dependent phospholipid scramblases that transport phospholipids bidirectionally, but its molecular mechanism is still unknown. Here we show that TMEM16F (transmembrane protein 16F) is an essential component for the Ca(2+)-dependent exposure of PtdSer on the cell surface. When a mouse B-cell line, Ba/F3, was treated with a Ca(2+) ionophore under low-Ca(2+) conditions, it reversibly exposed PtdSer. Using this property, we established a Ba/F3 subline that strongly exposed PtdSer by repetitive fluorescence-activated cell sorting. A complementary DNA library was constructed from the subline, and a cDNA that caused Ba/F3 to expose PtdSer spontaneously was identified by expression cloning. The cDNA encoded a constitutively active mutant of TMEM16F, a protein with eight transmembrane segments. Wild-type TMEM16F was localized on the plasma membrane and conferred Ca(2+)-dependent scrambling of phospholipids. A patient with Scott syndrome, which results from a defect in phospholipid scrambling activity, was found to carry a mutation at a splice-acceptor site of the gene encoding TMEM16F, causing the premature termination of the protein.

Targeting interferon response genes sensitizes aromatase inhibitor resistant breast cancer cells to estrogen-induced cell death.

INTRODUCTION: Estrogen deprivation using aromatase inhibitors (AIs) is currently the standard of care for postmenopausal women with hormone receptor-positive breast cancer. Unfortunately, the majority of patients treated with AIs eventually develop resistance, inevitably resulting in patient relapse and, ultimately, death. The mechanism by which resistance occurs is still not completely known, however, recent studies suggest that impaired/defective interferon signaling might play a role. In the present study, we assessed the functional role of IFITM1 and PLSCR1; two well-known interferon response genes in AI resistance. METHODS: Real-time PCR and Western blot analyses were used to assess mRNA and protein levels of IFITM1, PLSCR1, STAT1, STAT2, and IRF-7 in AI-resistant MCF-7:5C breast cancer cells and AI-sensitive MCF-7 and T47D cells. Immunohistochemistry (IHC) staining was performed on tissue microarrays consisting of normal breast tissues, primary breast tumors, and AI-resistant recurrence tumors. Enzyme-linked immunosorbent assay was used to quantitate intracellular IFNα level. Neutralizing antibody was used to block type 1 interferon receptor IFNAR1 signaling. Small interference RNA (siRNA) was used to knockdown IFITM1, PLSCR1, STAT1, STAT2, IRF-7, and IFNα expression. RESULTS: We found that IFITM1 and PLSCR1 were constitutively overexpressed in AI-resistant MCF-7:5C breast cancer cells and AI-resistant tumors and that siRNA knockdown of IFITM1 significantly inhibited the ability of the resistant cells to proliferate, migrate, and invade. Interestingly, suppression of IFITM1 significantly enhanced estradiol-induced cell death in AI-resistant MCF-7:5C cells and markedly increased expression of p21, Bax, and Noxa in these cells. Significantly elevated level of IFNα was detected in AI-resistant MCF-7:5C cells compared to parental MCF-7 cells and suppression of IFNα dramatically reduced IFITM1, PLSCR1, p-STAT1, and p-STAT2 expression in the resistant cells. Lastly, neutralizing antibody against IFNAR1/2 and knockdown of STAT1/STAT2 completely suppressed IFITM1, PLSCR1, p-STAT1, and p-STAT2 expression in the resistant cells, thus confirming the involvement of the canonical IFNα signaling pathway in driving the overexpression of IFITM1 and other interferon-stimulated genes (ISGs) in the resistant cells. CONCLUSION: Overall, these results demonstrate that constitutive overexpression of ISGs enhances the progression of AI-resistant breast cancer and that suppression of IFITM1 and other ISGs sensitizes AI-resistant cells to estrogen-induced cell death.

Phospholipid scramblase-1-induced lipid reorganization regulates compensatory endocytosis in neuroendocrine cells.

Calcium-regulated exocytosis in neuroendocrine cells and neurons is accompanied by the redistribution of phosphatidylserine (PS) to the extracellular space, leading to a disruption of plasma membrane asymmetry. How and why outward translocation of PS occurs during secretion are currently unknown. Immunogold labeling on plasma membrane sheets coupled with hierarchical clustering analysis demonstrate that PS translocation occurs at the vicinity of the secretory granule fusion sites. We found that altering the function of the phospholipid scramblase-1 (PLSCR-1) by expressing a PLSCR-1 calcium-insensitive mutant or by using chromaffin cells from PLSCR-1⁻/⁻ mice prevents outward translocation of PS in cells stimulated for exocytosis. Remarkably, whereas transmitter release was not affected, secretory granule membrane recapture after exocytosis was impaired, indicating that PLSCR-1 is required for compensatory endocytosis but not for exocytosis. Our results provide the first evidence for a role of specific lipid reorganization and calcium-dependent PLSCR-1 activity in neuroendocrine compensatory endocytosis.

Identification of key regulatory pathways of myeloid differentiation using an mESC-based karyotypically normal cell model.

Understanding the process of myeloid differentiation offers important insights into both normal and abnormal developmental processes but is limited by the dearth of experimental models. Here we show that myeloid progenitors can be derived from embryonic stem cells, immortalized, and applied to the study of the mechanisms underlying myeloid differentiation. The embryonic stem cell-derived myeloid progenitors, when immortalized with estrogen-regulated Hoxb8 protein, demonstrate normal karyotyping, are genetically tractable, and can be differentiated into functional neutrophils. Using this model, we identified mammalian target of rapamycin complex 1 as a critical regulator of myeloid differentiation. Together, our studies led to a convenient, karyotypically normal, and genetically manipulatable cellular system, which can be used to shed new light on the mechanisms for myeloid differentiation.

Phospholipid Scramblase 1, an interferon-regulated gene located at 3q23, is regulated by SnoN/SkiL in ovarian cancer cells.

BACKGROUND: Treatment of advanced stage ovarian cancer continues to be challenging due to acquired drug resistance and lack of early stage biomarkers. Genes identified to be aberrantly expressed at the 3q26.2 locus (i.e. SnoN/SkiL) have been implicated in ovarian cancer pathophysiology. We have previously shown that SnoN expression is increased in advanced stage ovarian cancers and alters cellular response to arsenic trioxide (As2O3). FINDINGS: We now demonstrate increased DNA copy number levels (TCGA data) of phospholipid scramblase 1 (PLSCR1, located at 3q23) whose transcript expression in ovarian cell lines is highly correlated with SnoN mRNA. Interestingly, SnoN can modulate PLSCR1 mRNA levels in the absence/presence of interferon (IFN-2α). Both IFN-2α and As2O3 treatment can modulate PLSCR1 mRNA levels in ovarian carcinoma cells. However, SnoN siRNA does not lead to altered PLSCR1 protein implicating other events needed to modulate its protein levels. In addition, we report that PLSCR1 can modulate aspects of the As2O3 cellular response. CONCLUSIONS: Our findings warrant further investigation into the role of PLSCR1 in ovarian cancer development and chemoresistance.

A minimal nuclear localization signal (NLS) in human phospholipid scramblase 4 that binds only the minor NLS-binding site of importin alpha1.

Importin α1 can bind classical nuclear localization signals (NLSs) in two NLS-binding sites, known as "major" and "minor." The major site is located between ARM repeats 2-4, whereas the minor site spans ARM 7-8. In this study, we have characterized the cellular localization of human phospholipid scramblase 4 (hPLSCR4), a member of the phospholipid scramblase protein family. We identified a minimal NLS in hPLSCR4 ((273)GSIIRKWN(280)) that contains only two basic amino acids. This NLS is both necessary for nuclear localization of hPLSCR4 in transfected HeLa cells and sufficient for nuclear import of a non-diffusible cargo in permeabilized cells. Mutation of only one of the two basic residues, Arg(277), correlates with loss of nuclear localization, suggesting this amino acid plays a key role in nuclear transport. Crystallographic analysis of mammalian importin α1 in complex with the hPLSCR4-NLS reveals this minimal NLS binds specifically and exclusively to the minor binding site of importin α. These data provide the first structural and functional evidence of a novel NLS-binding mode in importin α1 that uses only the minor groove as the exclusive site for nuclear import of nonclassical cargos.

Phosphatidylserine externalization and membrane blebbing are involved in the nonclassical export of FGF1.

The mechanisms of nonclassical export of signal peptide-less proteins remain insufficiently understood. Here, we demonstrate that stress-induced unconventional export of FGF1, a potent and ubiquitously expressed mitogenic and proangiogenic protein, is associated with and dependent on the formation of membrane blebs and localized cell surface exposure of phosphatidylserine (PS). In addition, we found that the differentiation of promonocytic cells results in massive FGF1 release, which also correlates with membrane blebbing and exposure of PS. These findings indicate that the externalization of acidic phospholipids could be used as a pharmacological target to regulate the availability of FGF1 in the organism.

Phospholipid scramblases and Tubby-like proteins belong to a new superfamily of membrane tethered transcription factors.

MOTIVATION: Phospholipid scramblases (PLSCRs) constitute a family of cytoplasmic membrane-associated proteins that were identified based upon their capacity to mediate a Ca(2+)-dependent bidirectional movement of phospholipids across membrane bilayers, thereby collapsing the normally asymmetric distribution of such lipids in cell membranes. The exact function and mechanism(s) of these proteins nevertheless remains obscure: data from several laboratories now suggest that in addition to their putative role in mediating transbilayer flip/flop of membrane lipids, the PLSCRs may also function to regulate diverse processes including signaling, apoptosis, cell proliferation and transcription. A major impediment to deducing the molecular details underlying the seemingly disparate biology of these proteins is the current absence of any representative molecular structures to provide guidance to the experimental investigation of their function. RESULTS: Here, we show that the enigmatic PLSCR family of proteins is directly related to another family of cellular proteins with a known structure. The Arabidopsis protein At5g01750 from the DUF567 family was solved by X-ray crystallography and provides the first structural model for this family. This model identifies that the presumed C-terminal transmembrane helix is buried within the core of the PLSCR structure, suggesting that palmitoylation may represent the principal membrane anchorage for these proteins. The fold of the PLSCR family is also shared by Tubby-like proteins. A search of the PDB with the HHpred server suggests a common evolutionary ancestry. Common functional features also suggest that tubby and PLSCR share a functional origin as membrane tethered transcription factors with capacity to modulate phosphoinositide-based signaling.

The negative c-Myc target onzin affects proliferation and apoptosis via its obligate interaction with phospholipid scramblase 1.

Onzin, the product of a negatively c-Myc-regulated target gene, is highly expressed in myeloid cells. As a result of its interaction with and activation of Akt1 and Mdm2, onzin down-regulates p53. The apoptotic sensitivity of several cell lines is thus directly related to onzin levels. We have conducted a search for additional onzin-interacting proteins and identified phospholipid scramblase 1 (PLSCR1), an endofacial membrane protein, which is proposed to mediate the bidirectional movement of plasma membrane phospholipids during proliferation and apoptosis. PLSCR1 interacts with the same cysteine-rich domain of onzin as do Akt1 and Mdm2, whereas the onzin-interacting domain of PLSCR1 centers around, but does not require, a previously identified palmitoylation signal. Depletion of endogenous PLSCR1 in myeloid cells leads to a phenotype that mimics that of onzin overexpression, providing evidence that PLSCR1 is a physiologic regulator of onzin. In contrast, PLSCR1 overexpression in fibroblasts, which normally do not express onzin, affects neither growth nor apoptosis unless onzin is coexpressed, in which case PLSCR1 completely abrogates onzin's positive effects on proliferation and survival. These findings demonstrate a functional interdependence between onzin and PLSCR1. They further suggest a contiguous link between the earliest events mediated by c-Myc and the latest ones, which culminate at the cell surface and lead to phospholipid reshuffling and cell death.

Human CD59 is a receptor for the cholesterol-dependent cytolysin intermedilysin.

Cholesterol is believed to serve as the common receptor for the cholesterol-dependent cytolysins (CDCs). One member of this toxin family, Streptococcus intermedius intermedilysin (ILY), exhibits a narrow spectrum of cellular specificity that is seemingly inconsistent with this premise. We show here that ILY, via its domain 4 structure, binds to the glycosyl-phosphatidylinositol-linked membrane protein human CD59 (huCD59). CD59 is an inhibitor of the membrane attack complex of human complement. ILY specifically binds to huCD59 via residues that are the binding site for the C8alpha and C9 complement proteins. These studies provide a new model for the mechanism of cellular recognition by a CDC.

Expression of the phospholipid scramblase (PLSCR) gene family during the acute phase response.

Phospholipid scramblase 1 (PLSCR1) is a member of PLSCR gene family that has been implicated in multiple cellular processes including movement of phospholipids, gene regulation, immuno-activation, and cell proliferation/apoptosis. In the present study, we identified PLSCR1 as a positive intracellular acute phase protein that is upregulated by LPS in liver, heart, and adipose tissue, but not skeletal muscle. LPS administration resulted in a marked increase in PLSCR1 mRNA and protein levels in the liver. This stimulation occurred rapidly (within 2 h), and was very sensitive to LPS (half-maximal response at 0.1 microg/mouse). Moreover, two other APR-inducers, zymosan and turpentine, also produced significant increases in PLSCR1 mRNA and protein levels, indicating that PLSCR1 was stimulated in a number of models of the APR. To determine signaling pathways by which LPS stimulated PLSCR1, we examined the effect of proinflammatory cytokines in vitro and in vivo. TNFalpha, IL-1beta, and IL-6 all stimulated PLSCR1 in cultured Hep B3 hepatocytes, whereas only TNFalpha stimulated PLSCR1 in cultured 3T3-L1 adipocytes, suggesting cell type-specific effects of cytokines. Furthermore, the LPS-stimulated increase in liver PLSCR1 mRNA was greatly attenuated by 80% in TNFalpha and IL-1beta receptor null mice as compared to wild-type controls. In contrast, PLSCR1 levels in adipose tissue were induced to a similar extent in TNFalpha and IL-1beta receptor null mice and controls. These results indicate that maximal stimulation of PLSCR1 by LPS in liver required TNFalpha and/or IL-1beta, whereas the stimulation of PLSCR1 in adipose tissue is not dependent on TNFalpha and/or IL-1beta. These data provide evidence that PLSCR1 is a positive intracellular acute phase protein with a tissue-specific mechanism for up-regulation.

Suppression of ovarian carcinoma cell growth in vivo by the interferon-inducible plasma membrane protein, phospholipid scramblase 1.

Phospholipid scramblase 1 (PLSCR1) is an IFN-inducible, endofacial plasma membrane protein that has been proposed to mediate accelerated transbilayer movement of plasma membrane phospholipids in cells exposed to elevated cytoplasmic [Ca (2+)]. The marked transcriptional up-regulation of this gene by IFN in a wide variety of cell types suggested that PLSCR1 might also contribute to biological effects associated with IFN. To study the potential contribution of cellular PLSCR1 to the antiproliferative and tumor-suppressive activities of IFN, PLSCR1 cDNA was stably expressed in the human ovarian cancer cell line HEY1B, and the growth of these cells was compared with matched vector transfected controls both in vitro and in vivo. Whereas we detected no difference in either growth rate or morphology between PLSCR1-transfected cells and vector controls during in vitro culture in serum, when these cells were implanted s.c. into athymic nude mice, we observed a marked suppression of tumor development from cells transfected to express elevated levels of PLSCR1. Tumors from the PLSCR1-transfected cells were greatly reduced in size, showed increased infiltration of leukocytes and macrophages, and appeared to undergo differentiation to a more uniform and spindle-shaped morphology that markedly contrasted the highly undifferentiated and pleiomorphic cell shape normally observed for HEY1B cells in vitro or for vector-transfected control HEY1B cells both in vitro and in vivo. These data suggest that the up-regulation of PLSCR1 expression in tumor cells exposed to IFN may contribute to the observed tumor-suppressive action of this cytokine.

Regulation of the tyrosine phosphorylation of Phospholipid Scramblase 1 in mast cells that are stimulated through the high-affinity IgE receptor.

Engagement of high-affinity immunoglobulin E receptors (FcεRI) activates two signaling pathways in mast cells. The Lyn pathway leads to recruitment of Syk and to calcium mobilization whereas the Fyn pathway leads to phosphatidylinositol 3-kinase recruitment. Mapping the connections between both pathways remains an important task to be completed. We previously reported that Phospholipid Scramblase 1 (PLSCR1) is phosphorylated on tyrosine after cross-linking FcεRI on RBL-2H3 rat mast cells, amplifies mast cell degranulation, and is associated with both Lyn and Syk tyrosine kinases. Here, analysis of the pathway leading to PLSCR1 tyrosine phosphorylation reveals that it depends on the FcRγ chain. FcεRI aggregation in Fyn-deficient mouse bone marrow-derived mast cells (BMMC) induced a more robust increase in FcεRI-dependent tyrosine phosphorylation of PLSCR1 compared to wild-type cells, whereas PLSCR1 phosphorylation was abolished in Lyn-deficient BMMC. Lyn association with PLSCR1 was not altered in Fyn-deficient BMMC. PLSCR1 phosphorylation was also dependent on the kinase Syk and significantly, but partially, dependent on detectable calcium mobilization. Thus, the Lyn/Syk/calcium axis promotes PLSCR1 phosphorylation in multiple ways. Conversely, the Fyn-dependent pathway negatively regulates it. This study reveals a complex regulation for PLSCR1 tyrosine phosphorylation in FcεRI-activated mast cells and that PLSCR1 sits at a crossroads between Lyn and Fyn pathways.

Nuclear phospholipid scramblase 1 prolongs the mitotic expansion of granulocyte precursors during G-CSF-induced granulopoiesis.

PLSCR1-/- mice exhibit normal, steady-state hematologic parameters but impaired emergency granulopoiesis upon in vivo administration of G-CSF. The mechanism by which PLSCR1 contributes to G-CSF-induced neutrophil production is largely unknown. We now report that the expansion of bone marrow myelocytes upon in vivo G-CSF treatment is reduced in PLSCR1-/- mice relative to WT. Using SCF-ER-Hoxb8-immortalized myeloid progenitors to examine the progression of G-CSF-driven granulocytic differentiation in vitro, we found that PLSCR1 prolongs the period of mitotic expansion of proliferative granulocyte precursors, thereby giving rise to increased neutrophil production from their progenitors. This effect of PLSCR1 is blocked by a ΔNLS-PLSCR1, which prevents its nuclear import. By contrast, mutation that prevents the membrane association of PLSCR1 has minimal impact on the role of PLSCR1 in G-CSF-induced granulopoiesis. These data imply that the capacity of PLSCR1 to augment G-CSF-dependent production of mature neutrophils from myeloid progenitors is unrelated to its reported activities at the endofacial surface of the plasma membrane but does require entry of the protein into the nucleus, suggesting that this response is mediated through the observed effects of PLSCR1 on gene transcription.

Mobilization of pro-inflammatory lipids in obese Plscr3-deficient mice.

BACKGROUND: The obesity epidemic has prompted the search for candidate genes capable of influencing adipose function. One such candidate, that encoding phospholipid scramblase 3 (PLSCR3), was recently identified, as genetic deletion of it led to lipid accumulation in abdominal fat pads and changes characteristic of metabolic syndrome. Because adipose tissue is increasingly recognized as an endocrine organ, capable of releasing small molecules that modulate disparate physiological processes, we examined the plasma from wild-type, Plscr1-/-, Plscr3-/- and Plscr1&3-/- mice. Using an untargeted comprehensive metabolite profiling approach coupled with targeted gene expression analyses, the perturbed biochemistry and functional redundancy of PLSCR proteins was assessed. RESULTS: Nineteen metabolites were differentially and similarly regulated in both Plscr3-/- and Plscr1&3-/- animals, of which five were characterized from accurate mass, tandem mass spectrometry data and their correlation to the Metlin database as lysophosphatidylcholine (LPC) species enriched with C16:1, C18:1, C20:3, C20:5 and C22:5 fatty acids. No significant changes in the plasma metabolome were detected upon elimination of PLSCR1, indicating that increases in pro-inflammatory lipids are specifically associated with the obese state of Plscr3-deficient animals. Correspondingly, increases in white adipose lipogenic gene expression confirm a role for PLSCR3 in adipose lipid metabolism. CONCLUSION: The untargeted profiling of circulating metabolites suggests no detectable functional redundancies between PLSCR proteins; however, this approach simultaneously identified previously unrecognized lipid metabolites that suggest a novel molecular link between obesity, inflammation and the downstream consequences associated with PLSCR3-deficiency.

Phospholipid scramblase 1 binds to the promoter region of the inositol 1,4,5-triphosphate receptor type 1 gene to enhance its expression.

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated, endofacial membrane protein originally identified based on its capacity to promote accelerated transbilayer phospholipid movement in response to Ca(2+). Recent evidence suggests that this protein also participates in cell response to various growth factors and cytokines, influencing myeloid differentiation, tumor growth, and the antiviral activity of interferon. Whereas plasma membrane PLSCR1 was shown to be required for normal recruitment and activation of Src kinase by stimulated cell surface growth factor receptors, PLSCR1 was also found to traffic into the nucleus and to tightly bind to genomic DNA, suggesting a possible additional nuclear function. We now report evidence that PLSCR1 directly binds to the 5'-promoter region of the inositol 1,4,5-triphosphate receptor type 1 gene (IP3R1) to enhance expression of the receptor. Probing a CpG island genomic library with PLSCR1 as bait identified four clones with avidity for PLSCR1, including a 191-bp fragment of the IP3R1 promoter. Using electrophoretic mobility shift and transcription reporter assays, the PLSCR1-binding site in IP3R1 was mapped to residues (-101)GTAACCATGTGGA(-89), and the segment spanning Met(86)-Glu(118) in PLSCR1 was identified to mediate its transcriptional activity. The significance of this interaction between PLSCR1 and IP3R1 in situ was confirmed by comparing levels of IP3R1 mRNA and protein in matched cells that either expressed or were deficient in PLSCR1. These data suggest that in addition to its role at the plasma membrane, effects of PLSCR1 on cell proliferative and maturational responses may also relate to alterations in expression of cellular IP3 receptors.

Interferon-alpha-induced expression of phospholipid scramblase 1 through STAT1 requires the sequential activation of protein kinase Cdelta and JNK.

Phospholipid scramblase 1 (PLSCR1), a calcium-binding protein that either inserts into the plasma membrane or binds to genomic DNA in the nucleus, has been shown to contribute to the cell proliferation, differentiation, and apoptosis as well as antiviral activity of interferon (IFN). The expression of PLSCR1 protein is also known to be markedly increased in response to IFN and to some differentiation inducing agents such as all-trans retinoic acid, but the precise mechanisms of this response remain to be investigated. In this study, we show that the protein kinase Cdelta (PKCdelta)-specific inhibitor rottlerin and the dominant negative mutant of PKCdelta significantly antagonized IFN-induced PLSCR1 expression. The influence of PKCdelta on IFN-mediated induction of PLSCR1 was dependent upon the phosphorylation of STAT1 at Ser-727. Furthermore, PKCdelta-mediated activation of STAT1 required the activation of JNK, as the inhibition of JNK activity by its specific inhibitor or transfection of its dominant negative mutant suppressed both serine phosphorylation of STAT1 and PLSCR1 expression but not the activation of PKCdelta. In conclusion, our results suggest that the induction of PLSCR1 transcription through STAT1 depends upon sequential activation of PKCdelta and JNK.

Phospholipid scramblase 1 contains a nonclassical nuclear localization signal with unique binding site in importin alpha.

Nuclear import of proteins containing a classical nuclear localization signal (NLS) is an energy-dependent process that requires the heterodimer importin alpha/beta. Three to six basic contiguous arginine/lysine residues characterize a classical NLS and are thought to form a basic patch on the surface of the import cargo. In this study, we have characterized the NLS of phospholipid scramblase 1 (PLSCR1), a lipid-binding protein that enters the nucleus via the nonclassical NLS (257)GKISKHWTGI(266). This import sequence lacks a contiguous stretch of positively charged residues, and it is enriched in hydrophobic residues. We have determined the 2.2 A crystal structure of a complex between the PLSCR1 NLS and the armadillo repeat core of vertebrate importin alpha. Our crystallographic analysis reveals that PLSCR1 NLS binds to armadillo repeats 1-4 of importin alpha, but its interaction partially overlaps the classical NLS binding site. Two PLSCR1 lysines occupy the canonical positions indicated as P2 and P5. Moreover, we present in vivo evidence that the critical lysine at position P2, which is essential in other known NLS sequences, is dispensable in PLSCR1 NLS. Taken together, these data provide insight into a novel nuclear localization signal that presents a distinct motif for binding to importin alpha.

Normal hemostasis but defective hematopoietic response to growth factors in mice deficient in phospholipid scramblase 1.

Phospholipid scramblase 1 (PLSCR1) is an endofacial plasma membrane protein proposed to participate in transbilayer movement of phosphatidylserine and other phospholipids. In addition to its putative role in the reorganization of plasma membrane phospholipids, PLSCR1 is a substrate of intracellular kinases that imply its possible participation in diverse signaling pathways underlying proliferation, differentiation, or apoptosis. Because PLSCR1 is prominently expressed in a variety of blood cells, we evaluated PLSCR activity in platelets and erythrocytes, and cytokine-dependent growth of hematopoietic precursor cells, of PLSCR1 knock-out mice. Adult PLSCR1(-/-) mice showed no obvious hematologic or hemostatic abnormality, and blood cells from these animals normally mobilized phosphatidylserine to the cell surface upon stimulation. Whereas blood cell counts in adult PLSCR1(-/-) mice were normal, in both fetus and newborn animals neutrophil counts were significantly depressed relative to age-matched wild type (WT). Furthermore, when compared with WT, hematopoietic precursor cells from PLSCR1(-/-) mice showed defective colony formation and impaired differentiation to mature granulocytes as stimulated by stem cell factor and granulocyte colony-stimulating factor (G-CSF). By contrast, PLSCR1(-/-) cells showed normal colony formation stimulated by interleukin-3 or granulocyte-macrophage CSF, and expansion of megakaryocytic and erythroid progenitors by thrombopoietin or erythropoietin was unaffected. Stem cell factor and G-CSF were also found to induce marked increases in PLSCR1 levels in WT cells. Consistent with in vitro assays, PLSCR1(-/-) mice treated with G-CSF showed less than 50% of the granulocytosis observed in identically treated WT mice. These data provide direct evidence that PLSCR1 functionally contributes to cytokine-regulated cell proliferation and differentiation and suggest it is required for normal myelopoiesis.