David Sabatini--a lifelong fascination with organelles

As a pioneer molecular cell biologist, highly skilled in both morphological and biochemical approaches, David Sabatini was a key figure in laying the foundation for the field of intracellular protein trafficking with his seminal studies on cotranslational translocation of nascent polypeptides in the endoplasmic reticulum and the intracellular sorting of plasma membrane proteins in polarized epithelial cells.     

Malaria: endless fascination with merozoite release

The erythrocytic asexual reproduction cycle of Plasmodium falciparum, the most lethal malaria parasite in humans, starts with the invasion of a red blood cell by a merozoite and ends with the release of up to 32 new copies of itself in about 48 hours. A new study reveals that merozoite release is an explosive event ensuring the dispersal of these non-motile parasites for optimal re-invasion of new red cells.     

Interactions of sugars with membranes

Water profoundly affects the stability of biological membranes, and its removal leads to destructive events including fusion and liquid crystalline to gel phase transitions. In heterogeneous mixtures such as those found in biological membranes the phase transitions can lead to increases in permeability and lateral phase separations that often are irreparable. Certain sugars are capable of preventing these deleterious events by inhibiting fusion during drying and by maintaining the lipid in a fluid state in the absence of water. As a result, the increased permeability and lateral phase separations that accompany dehydration are absent. The weight of the evidence suggests strongly that there is a direct interaction between the sugars and lipids in the dry state. Although the evidence is less clear about whether these sugars can interact directly with hydrated bilayers, there are strong suggestions in the literature that sugars free in solution or covalently linked to membrane constituents can also affect the physical properties and presumably the stability of bilayers. Finally, we have far less evidence concerning the mechanism by which they do so, but the same sugars are also capable of preserving the structure and function of both membrane-bound and soluble proteins in the absence of water. We believe these effects may be important in the survival of intact cells and organisms such as seeds in the absence of water. Furthermore, in view of the practical importance of preserving biological structures we suspect that the results described here will ultimately have important applications in biology and medicine.     

Syntheses of branched-chain sugars with push-pull functionality

The reaction of methyl 4,6-O-benzylidene-3(2)-deoxy-- -erythro-hexopyranosid-2(3)-ulose with carbon disulfide, alkyl iodide, and sodium hydride gave methyl 4,6-O-benzylidene-3(2)-[bis(alkylthio)methylene]-3(2)-deoxy-- -erythro-hexopyranosid-2(3)-uloses. Methyl 4,6-O-benzylidene-2-[bis(methylthio)methylene]-2-deoxy-- -erythro-hexopyranosid-3-ulose (5) reacted with aromatic amines to give, in a rearrangement process, N-aryl-2-aryliminomethyl-4,6-O-benzylidene-2-deoxy-- -erythro-hex-1-enopyranosylamin-3-uloses. The reaction of 5 which hydrazine hydrate afforded 5-methylthio-(methyl-4,6-O-benzylidene-2,3-dideoxy-- -erythro-hexopyranosido)[3,2-c]pyrazole.     

Glycosylation reaction of unprotected sugars with hydroxyalkylthymine

Under mild conditions (Lewis acid/solvent/room temperature), the reaction of unprotected glucose, deoxyribose or xylose with hydroxylalkylthymine gives selectively nucleoside analogs with a spacer arm between sugar and base moiety. Experimental conditions (Lewis acid, solvent) for this new strategy leading to nucleoside analogs synthesis are discussed.     

Chain elongation of sugars with ethyl isocyanoacetate

Addition of ethyl isocyanoacetate to 3-O-benzyl-1,2-O-isopropylidene-α-D-ribo-pentodialdo-1,4-furanose in ethanolic sodium cyanide gave two oxazolines that were hydrolysed during chromatography to two isomeric ethyl 3-O-benzyl-6-deoxy-6-formamido-1,2-O-isopropylidene-heptofuranuronates. Similarly, 1,2-O-isopropyl-idene-3-O-methyl-α-D-xylo-pentodialdo-1,4-furanose gave the 3-O-methyl-heptofuranuronates 7 and 11. Reduction of 7 and 11 gave N-methylamino esters that exhibited Cotton effects from which the configurations at C-6 of 7 and 11 were deduced. The chiralities at C-5 of 7 and 11 were established by tetrahydropyranlation of 7 and 11, followed by consecutive treatment with bis(2-methoxyethoxy)aluminium hydride, periodate, sodium borohydride, and dilute acid, to give 1,2-O-isopropylidene-3-O-methyl-α-D-glucofuranose and its β-L-ido epimer, respectively. Attempts to methylate HO-5 of 7 and 11 resulted in elimination. On formylaminomethylenation (ethyl isocyanoacetate and potassium hydride in tetrahydrofuran), 3-O-benzyl-1,2-O-isopropylidene-α-D-ribo-pentodialdo-1,4-furanose and its 3-O-methyl-α-D-xylo epimer each gave (E)- and (Z)-mixtures of alkenes that were hydrogenated to give mixtures of 5,6-dideoxy-6-formamido-heptofuranuronates.     

Melittin interaction with sulfated cell surface sugars

Melittin is a 26-residue cationic peptide with cytolytic and antimicrobial properties. Studies on the action mechanism of melittin have focused almost exclusively on the membrane-perturbing properties of this peptide, investigating in detail the melittin-lipid interaction. Here, we report physical-chemical studies on an alternative mechanism by which melittin could interact with the cell membrane. As the outer surface of many cells is decorated with anionic (sulfated) glycosaminoglycans (GAGs), a strong Coulombic interaction between the two oppositely charged molecules can be envisaged. Indeed, the present study using isothermal titration calorimetry reveals a high affinity of melittin for several GAGs, that is, heparan sulfate (HS), dermatan sulfate, and heparin. The microscopic binding constant of melittin for HS is 2.4 x 10 (5) M (-1), the reaction enthalpy is Delta H melittin (0) = -1.50 kcal/mol, and the peptide-to-HS stoichiometry is approximately 11 at 10 mM Tris, 100 mM NaCl at pH 7.4 and 28 degrees C. Delta H melittin (0) is characterized by a molar heat capacity of Delta C P (0) = -227 cal mol (-1) K (-1). The large negative heat capacity change indicates that hydrophobic interactions must also be involved in the binding of melittin to HS. Circular dichroism spectroscopy demonstrates that the binding of the peptide to HS induces a conformational change to a predominantly alpha-helical structure. A model for the melittin-HS complex is presented. Melittin binding was compared with that of magainin 2 and nisin Z to HS. Magainin 2 is known for its antimicrobial properties, but it does not cause lysis of the eukaryotic cells. Nisin Z shows activity against various Gram-positive bacteria. Isothermal titration calorimetry demonstrates that magainin 2 and nisin Z do not bind to HS (5-50 degrees C, 10 mM Tris, and 100 mM NaCl at pH 7.4).     

Sulfotransferases, sulfatases and formylglycine-generating enzymes: a sulfation fascination

Sulfotransferases and sulfatases are the major enzymes responsible for sulfate transfer processes. The past two years have seen the elucidation of new functions for these enzymes, and a great progression in their structural characterization, which confirms that these two types of enzymes possess a highly conserved fold. For catalytic activity, sulfatases must contain a formylglycine residue, which is generated by various formylglycine-generating enzymes. Mechanistic and structural details have recently been obtained for a group of cofactor-independent formylglycine-generating enzymes termed FGEs. Finally, an increasing light has been cast upon the mechanism of sulfatase inactivation by a group of clinically important agents, the aryl sulfamates.     

A sensitive fluorometric assay for reducing sugars.

A simple and rapid fluorometric assay for reducing sugars that is sensitive to the nanomolar range has been developed. The assay involves the derivatization of a given sugar with hydrazine at pH 3 to form a hydrazone, which is reacted with fluorescamine following adjustment of pH to first 9.4 and then 7.4. The amount of sugar in a sample is quantitated by measuring the fluorescence intensity at an excitation wavelength of 400 nm and an emission wavelength of 490 nm. The assay is precise and reproducible, as indicated by intra- and inter-run variations of at most 3% and 4%, respectively. In addition to reducing sugars, the assay can also be used to measure aliphatic and aromatic aldehydes, but not acetone. Compared with an existing fluorometric sugar assay, the assay reported here does not require chromatographic separation of the fluorescent derivative from unreacted fluorescamine. The assay can, however, be potentially adapted for postcolumn detection of aldehydes, reducing sugars, and hydrazones in HPLC.     

Fluorometric analysis of amino sugars and derivatized neutral sugars

A rapid and sensitive procedure for the analysis of neutral and amino sugars is presented. Neutral sugars are separated after conversion to the corresponding glycamines, while the amino sugars are analyzed without modification, using an automatic amino acid analyzer and fluorometric detection. The method has been applied for the analysis of glycoproteins and oligosaccharides of the complex and high-mannose types.     

Glycolipid transfer protein from pig brain transfers glycolipids with beta-linked sugars but not with alpha-linked sugars at the sugar-lipid linkage

The glycolipid transfer protein purified from pig brain facilitates the transfer of various glycosphingolipids and glyceroglycolipids (Yamada, K., Abe, A. and Sasaki, T. (1985) J. Biol. Chem. 260, 4615-4621). In this paper, the transfer of Man beta 1----4Glc beta 1-Cer and Man alpha 1----4Man beta 1-Cer isolated from a bivalve, Corbicula japonica, the transfer of 3-[Glc alpha 1-]-sn-1,2-diacylglycerol and 3-[Glc alpha 1----2Glc alpha 1-]-sn-1,2-diacylglycerol prepared from Streptococcus lactis, and the transfer of 3-[Glc beta 1-]-rac-1,2-dipalmitylglycerol have been investigated. The transfer of these lipids from liposomes to mitochondria was assayed by the decrease of these lipids in the donor liposomes. These lipids were determined by chromatographic isolation of the lipids, acid hydrolysis of the isolated lipids, and subsequent determination of glucose in the hydrolysate. The glycolipid transfer protein facilitated the transfer of ManGlcCer and ManManGlcCer. The transfer protein did not facilitate the transfer of Glc alpha-diacylglycerol or Glc alpha Glc alpha-diacylglycerol. However, the transfer of Glc beta-dipalmitylglycerol was facilitated by the protein. These results strongly suggest that the glycolipid transfer protein has the specificity to the presence of beta-linked glucose or galactose directly linked to either ceramide or diacylglycerol.     

Nonenzymatic glycation of DNA nucleosides with reducing sugars

Reducing sugars can react with the free amino groups of proteins to form a heterogeneous group of compounds known as advanced glycation endproducts (AGEs) or Maillard reaction products. The objective of this investigation was to monitor the nonenzymatic glycation of DNA nucleosides and to characterize the formation of nucleoside AGEs using capillary electrophoresis (CE), high-performance liquid chromatography (HPLC), UV fluorescence spectroscopy, and mass spectrometry. Deoxyguanosine, deoxyadenosine, deoxythymidine, and deoxycytidine were used as the model nucleosides and were incubated over time with glucose, galactose, or glyceraldehyde. Under increasing concentrations and time, deoxyguanosine exhibited the highest rate of glycation with glyceraldehyde. Deoxyadenosine and deoxycytidine exhibited comparable reactivity with glyceraldehyde and no appreciable reactivity with galactose or glucose. No reactivity was observed between deoxythymidine and the sugars. A combination of CE, HPLC, UV fluorescence spectroscopy, and mass spectrometry provided a convenient method for characterizing nucleoside AGEs and for monitoring the physical factors that influence the formation of sugar adducts of DNA nucleosides.