Sphingolipid photoaffinity labels

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

Gangliosides are important for the preservation of the structure and organization of RBL-2H3 mast cells

Gangliosides are known to be important in many biological processes. However, details concerning the exact function of these glycosphingolipids in cell physiology are poorly understood. In this study, the role of gangliosides present on the surface of rodent mast cells in maintaining cell structure was examined using RBL-2H3 mast cells and two mutant cell lines (E5 and D1) deficient in the gangliosides, GM(1) and the alpha-galactosyl derivatives of the ganglioside GD(1b). The two deficient cell lines were morphologically different from each other as well as from the parental RBL-2H3 cells. Actin filaments in RBL-2H3 and E5 cells were under the plasma membrane following the spindle shape of the cells, whereas in D1 cells, they were concentrated in large membrane ruffles. Microtubules in RBL-2H3 and E5 cells radiated from the centrosome and were organized into long, straight bundles. The bundles in D1 cells were thicker and organized circumferentially under the plasma membrane. The endoplasmic reticulum, the Golgi complex, and the secretory granule matrix were also altered in the mutant cell lines. These results suggest that the mast cell-specific alpha-galactosyl derivatives of ganglioside GD(1b) and GM(1) are important in maintaining normal cell morphology.

Protein-sphingolipid interactions within cellular membranes

Each intracellular organelle critically depends on maintaining its specific lipid composition that in turn contributes to the biophysical properties of the membrane. With our knowledge increasing about the organization of membranes with defined microdomains of different lipid compositions, questions arise regarding the molecular mechanisms that underlie the targeting to/segregation from microdomains of a given protein. In addition to specific lipid-transmembrane segment interactions as a basis for partitioning, the presence in a given microdomain may alter the conformation of proteins and, thus, the activity and availability for regulatory modifications. However, for most proteins, the specific lipid environment of transmembrane segments as well as its relevance to protein function and overall membrane organization are largely unknown. To help fill this gap, we have synthesized a novel photoactive sphingolipid precursor that, together with a precursor for phosphoglycerolipids and with photo-cholesterol, was investigated in vivo with regard to specific protein transmembrane span-lipid interactions. As a proof of principle, we show specific labeling of the ceramide transporter with the sphingolipid probe and describe specific in vivo interactions of lipids with caveolin-1, phosphatidylinositol transfer protein beta, and the mature form of nicastrin. This novel photolabile sphingolipid probe allows the detection of protein-sphingolipid interactions within the membrane bilayer of living cells.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Photoaffinity labelling of the Human GM2-activator protein. Mechanistic insight into ganglioside GM2 degradation

The GM2-activator protein (GM2AP) is an essential cofactor for the degradation of ganglioside GM2 by lysosomal beta-hexosaminidase A. It mediates the interaction between the water-soluble exohydrolase and its membrane-bound substrate at the lipid-water interphase. Inherited defects in the gene encoding this glycoprotein result in a fatal neurological storage disorder, the AB variant of GM2-gangliosidosis. To elucidate the mode of action of this glycoprotein cofactor, we synthesized the two photoaffinity labels [14C]C3-TPD-GM2 and [14C]C7-TPD-GM2. Incubation of GM2AP with these substrate analogues and subsequent irradiation led to covalent labelling of the protein. After separation of tryptic peptides by reverse-phase HPLC, the labelled peptide fractions were analysed by MALDI-TOF and sequenced by ESI-Q-TOF mass spectrometry. Both labels were found to be specifically photoincorporated into a part of the surface loop comprising residues V153-L163, a stretch of amino acids that was previously identified as the most flexible region in the crystal structure of the activator. Our results provide strong evidence that this loop constitutes the part of the activator protein that directly interacts with the ganglioside substrate, suggesting that the hydrophobicity and the great structural mobility of this element are crucial for the extraction of the membrane-embedded glycolipid, its stabilization inside the spacious cavity and its guidance to the enzyme's active site. This study demonstrates that the approach of photoaffinity labelling in conjunction with accurate mass measurements can provide insight into substrate binding interactions that complements structural information.

Versatile synthesis of phenoxydiazirine-based fatty acid analogues and photoreactive galactosylceramide

A versatile synthesis of diazirine-based photoreactive fatty acid analogues is reported. The key step is phenoxy alkylation of diazirine with halo alkyl acid esters. The conditions described will be acceptable for the synthesis of various alkyl-length derivatives. The fatty acid derivatives are acceptors for reverse reactions of sphingolipid ceramide N-deacylase (SCDase), which catalyzes the condensation of psychosine and fatty acids to form photoreactive galactosylceramide. The photoreactive galactosylceramide can also be prepared with chemical synthesis, condensation of psychosine and fatty acid succinimidyl ester, and is recognized with anti-GarCer antibody both before and after irradiation.