Phosphorylated sphingolipids [ceramide-1-phosphate (C1P) and sphingosine-1-phosphate (S1P)] have emerged as key regulators of cell growth, survival, migration, and inflammation1–5. C1P (Fig. 1a) produced by ceramide kinase is an activator of group IVA cytosolic phospholipase A2α (cPLA2α), the rate-limiting releaser of arachidonic acid used for pro-inflammatory eicosanoid production3,6–9, which contributes to disease pathogenesis in asthma/airway hyper-responsiveness, cancer, atherosclerosis, and thrombosis. To modulate eicosanoid action and avoid the damaging effects of chronic inflammation, cells require efficient targeting, trafficking, and presentation of C1P to specific cellular sites. Vesicular trafficking is likely10 but nonvesicular mechanisms for C1P sensing, transfer, and presentation remain unexplored11,12. Moreover, the molecular basis for selective recognition and binding among signaling lipids with phosphate headgroups, namely C1P, phosphatidic acid (PA) or their lyso-derivatives, remains unclear. Herein, an ubiquitously-expressed lipid transfer protein (CPTP) is shown to specifically transfer C1P between membranes. Crystal structures establish C1P binding via a novel surface-localized, phosphate headgroup recognition center connected to an interior hydrophobic pocket that adaptively expands to ensheath differing-length lipid chains using a cleft-like gating mechanism. The two-layer, α-helically-dominated ‘sandwich’ topology identifies CPTP as the prototype for a new GLTP-fold13 subfamily. CPTP resides in the cell cytosol but associates with the trans-Golgi/TGN, nucleus, and plasma membrane. RNAi-induced CPTP depletion elevates C1P steady-state levels and alters Golgi cisternae stack morphology. The resulting C1P decrease in plasma membranes and increase in the Golgi complex stimulates cPLA2α release of arachidonic acid, triggering pro-inflammatory eicosanoid generation.

Resonance energy transfer between anthrylvinyl-labeled phosphatidylcholine as a donor and heme moiety of cytochrome c (cyt c) as an acceptor has been employed to explore the protein binding to model membranes, composed of phosphatidylcholine and cardiolipin (CL). The existence of two types of protein-lipid complexes has been hypothesized where either deprotonated or partially protonated CL molecules are responsible for cyt c attachment to bilayer surface. To quantitatively describe cyt c membrane binding, the adsorption model based on scaled particle and double layer theories has been employed, with potential-dependent association constants being treated as a function of acidic phospholipid mole fraction, degree of CL protonation, ionic strength, and surface coverage. Multiple arrays of resonance energy transfer data obtained under conditions of varying pH, ionic strength, CL content, and protein/lipid molar ratio have been analyzed in terms of the model of energy transfer in two-dimensional systems combined with the adsorption model allowing for area exclusion and electrostatic effects. The set of recovered model parameters included effective protein charge, intrinsic association constants, and heme distance from the bilayer midplane for both types of protein-lipid complexes. Upon increasing CL mole fraction from 10 to 20 mol % (the value close to that characteristic of the inner mitochondrial membrane), the binding equilibrium dramatically shifted toward cyt c association with partially protonated CL species. The estimates of heme distance from bilayer center suggest shallow bilayer location of cyt c at physiological pH, whereas at pH below 6.0, the protein tends to insert into membrane core.

SUMMARY The accelerated-cell-death11 (acd11) mutant of Arabidopsis provides a genetic model for studying immune response activation and localized cellular suicide that halts pathogen spread during infection in plants. Here, we elucidate ACD11 structure/function and show that acd11 disruption dramatically alters the in vivo balance of sphingolipid mediators that regulate eukaryotic programmed cell death. In acd11 mutants, normally low ceramide-1-phosphate (C1P) levels become elevated, but the relatively abundant cell death inducer, phytoceramide, rises acutely. ACD11 exhibits selective intermembrane transfer of C1P and phyto-C1P. Crystal structures establish C1P binding via a surface-localized, phosphate headgroup recognition center connected to an interior hydrophobic pocket that adaptively ensheaths lipid chains via a cleft-like gating mechanism. Point mutation mapping confirms functional involvement of binding-site residues. A π-helix (π-bulge) near the lipid-binding cleft distinguishes apo-ACD11 from other GLTP-folds. The global two-layer, α-helically-dominated, ‘sandwich’ topology displaying C1P-selective binding identifies ACD11 as the plant prototype of a new GLTP-fold subfamily.

Many fluorescent lipid probes tend to loop back to the membrane interface when attached to a lipid acyl chain rather than embedding deeply into the bilayer. To achieve maximum embedding of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorophore into the bilayer apolar region, a series of sn-2 acyl-labeled phosphatidylcholines was synthesized bearing 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me 4 -BODIPY-8) at the end of C 3 -, C 5 -, C 7 -, or C 9 -acyl. A strategy was used of symmetrically dispersing the methyl groups at BODIPY ring positions 1, 3, 5, and 7 to decrease fluorophore polarity. Iodide quenching of the phosphatidylcholine probes in bilayer vesicles confirmed that the Me 4 -BODIPY-8 fluorophore was embedded in the bilayer. Parallax analysis of Me 4 -BODIPY-8 fluorescence quenching by phosphatidylcholines containing iodide at different positions along the sn-2 acyl chain indicated that the penetration depth of Me 4 -BODIPY-8 into the bilayer was determined by the length of the linking acyl chain. Evaluation using monolayers showed minimal perturbation of ,10 mol% probe in fluid-phase and cholesterol-enriched phosphatidylcholine. Spectral characterization in monolayers and bilayers confirmed the retention of many features of other BODIPY derivatives (i.e., absorption and emission wavelength maxima near 498 nm and ?506-515 nm) but also showed the absence of the 620-630 nm peak associated with BODIPY dimer fluorescence and the presence of a 570 nm emission shoulder at high Me 4 -BODIPY-8 surface concentrations. We conclude that the new probes should have versatile utility in membrane studies, especially when precise location of the reporter group is needed. Fluorescent lipid probes have proven to be valuable tools in membrane studies (see Ref. 1 for review). Because the determination of depth-dependent parameters of bilayers can benefit the understanding of membranous structures (2), sets of probes bearing the same fluorophore at different distances from the bilayer surface are potentially quite useful. Ideally, such fluorophores should be apolar enough to localize at the membrane depth that reflects the apolar nature of the surrounding acyl chain region without being strongly influenced by the transbilayer polarity gradient (3). The first probe set designed to achieve this goal was a series of n-(9-anthroyloxy) fatty acids synthesized by Thulborn and Sawyer (4), who showed that the fluorophore resided in the bilayer at a graded series of depths that coincided with the attachment point of the anthroyloxy fluorophore along the acyl chain. Other widely used fluorophores, such as N-dansyl (5) or N-NBD (6, 7), have been shown to have polar characteristics that interfere with localization deep inside the bilayer even when attached to the end of the acyl chain.During the past decade, BODIPY (4,4-difluoro-4-bora3a,4a-diaza-s-indacene) fluorophore probes have found a wide range of applications in cell biology and biophysics, even though this zwitterionic fluorophore wa...

SUMMARY Human glycolipid transfer protein (GLTP) fold represents a novel structural motif for lipid binding/transfer and reversible membrane translocation. GLTPs transfer glycosphingolipids (GSLs) which are key regulators of cell growth, division, surface adhesion, and neurodevelopment. Herein, we report structure-guided engineering of the lipid binding features of GLTP. New crystal structures of wild-type GLTP and two mutants (D48V and A47D||D48V), each containing bound N-nervonoyl-sulfatide, reveal the molecular basis for selective anchoring of sulfatide (3-O-sulfo-galactosylceramide) by D48V-GLTP. Directed point mutations of ‘portal entrance’ residues, A47 and D48, reversibly regulate sphingosine access to the hydrophobic pocket via a mechanism that could involve homo-dimerization. ‘Door-opening’ conformational changes by phenylalanines within the hydrophobic pocket are revealed during lipid encapsulation by new crystal structures of bona fide apo-GLTP and GLTP complexed with N-oleoyl-glucosylceramide. The development of ‘engineered GLTPs’ with enhanced specificity for select GSLs provides a potential new therapeutic approach for targeting GSL-mediated pathologies.