The cellular target of leptomycin B (LMB), a nuclear export inhibitor, has been identified as CRM1 (exportin 1), an evolutionarily conserved receptor for the nuclear export signal of proteins. However, the mechanism by which LMB inhibits CRM1 still remains unclear. CRM1 in a Schizosaccharomyces pombe mutant showing extremely high resistance to LMB had a single amino acid replacement at Cys-529 with Ser. The mutant gene, named crm1-K1, conferred LMB resistance on wild-type S. pombe, and Crm1-K1 no longer bound biotinylated LMB. 1 H NMR analysis showed that LMB bound N-acetyl-L-cysteine methyl ester through a Michaeltype addition, consistent with the idea that LMB binds covalently via its ␣,␤-unsaturated ␦-lactone to the sulfhydryl group of Cys-529. When HeLa cells were cultured with biotinylated LMB, the only cellular protein bound covalently was CRM1. Inhibition by N-ethylmaleimide (NEM), an alkylating agent, of CRM1-mediated nuclear export probably was caused by covalent binding of the electrophilic structure in NEM to the sulfhydryl group of Cys-529, because the crm1-K1 mutant showed the normal rate for the export of Rev nuclear export signal-bearing proteins in the presence of not only LMB but also NEM. These results show that the single cysteine residue determines LMB sensitivity and is selectively alkylated by LMB, leading to CRM1 inactivation.Many cellular proteins either reside in the nucleus or shuttle between the nucleus and the cytoplasm by energy-dependent transport across the nuclear envelope. Specific sequences within a protein contain the information necessary for the nucleocytoplasmic transport: most nuclear proteins have nuclear localization sequences (NLS) rich in basic amino acids, whereas others carry short nuclear export sequences (NES) rich in leucine (1, 2). CRM1͞exportin 1 was shown to be a receptor for the NES in both lower and higher eukaryotes (3-6). Genetic alterations in the CRM1 locus caused a defect in nuclear export of NES-bearing proteins in yeast (3,5,7,8). Nuclear microinjection of a CRM1-specific antibody that prevents the in vitro NES binding inhibited in vivo protein nuclear export in mammalian cells (9). Thus, the NESmediated nuclear export of proteins is a universal and conserved mechanism by which subcellular localization of proteins is controlled in cells.CRM1 originally was identified as a protein essential for maintaining chromosome structure in the fission yeast Schizosaccharomyces pombe (10). The functional homologues that complement the fission yeast crm1 mutation were cloned from the budding yeast Saccharomyces cerevisiae (11) and from human cells (8,12). We showed previously that a mutation (crm1-N1) of S. pombe crm1 ϩ conferred resistance to leptomycin B (LMB) (13), which had been discovered as a potent antifungal antibiotic blocking the eukaryotic cell cycle (14, 15). In contrast, the cold-sensitive crm1-809 mutant strain was hypersensitive to LMB. Furthermore, morphological and biochemical phenotypes of crm1-809 mutant cells at nonpermissive temperature ...

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Increased expression of vascular cell adhesion molecule 1 (VCAM1) is associated with a variety of chronic inflammatory conditions, making its expression and function a target for therapeutic intervention. We have recently identified CAM741, a derivative of a fungus-derived cyclopeptolide that acts as a selective inhibitor of VCAM1 synthesis in endothelial cells. Here we show that the compound represses the biosynthesis of VCAM1 in cells by blocking the process of cotranslational translocation, which is dependent on the signal peptide of VCAM1. CAM741 does not inhibit targeting of the VCAM1 nascent chains to the translocon channel but prevents translocation to the luminal side of the endoplasmic reticulum (ER), through a process that involves the translocon component Sec61beta. Consequently, the VCAM1 precursor protein is synthesized towards the cytosolic compartment of the cells, where it is degraded. Our results indicate that the inhibition of cotranslational translocation with low-molecular-mass compounds, using specificity conferred by signal peptides, can modulate the biosynthesis of certain secreted and/or membrane proteins. In addition, they highlight cotranslational translocation at the ER membrane as a potential target for drug discovery.

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The cyclopeptolide CAM741 inhibits cotranslational translocation of vascular cell adhesion molecule 1 (VCAM1), which is dependent on its signal peptide. We now describe the identification of the signal peptide of vascular endothelial growth factor (VEGF) as the second target of CAM741. The mechanism by which the compound inhibits translocation of VEGF is very similar or identical to that of VCAM1, although the signal peptides share no obvious sequence similarities. By mutagenesis of the VEGF signal peptide, two important regions, located in the N-terminal and hydrophobic segments, were identified as critical for compound sensitivity. CAM741 alters positioning of the VEGF signal peptide at the translocon, and increasing hydrophobicity in the h-region reduces compound sensitivity and causes a different, possibly more efficient, interaction with the translocon. Although CAM741 is effective against translocation of both VEGF and VCAM1, the derivative NFI028 is able to inhibit only VCAM1, suggesting that chemical derivatization can alter not only potency, but also the specificity of the compounds.We have recently reported that the cyclopeptolide CAM741, a derivative of the naturally occurring substance Hun-7293, inhibits cotranslational translocation of vascular cell adhesion molecule 1 (VCAM1), which is dependent on its signal peptide (SP) and occurs at the level of VCAM1 SP insertion in the Sec61 translocon (Besemer et al., 2005;Harant et al., 2006). Very similar observations were reported by Garrison et al. (2005) using cotransin, a compound with related structure. These findings demonstrate for the first time that a compound can interfere with the process of cotranslational translocation in a SP-dependent manner.Amino-terminal, cleavable SPs, when emerging from the ribosome, are recognized by the signal recognition particle, which then directs the ribosome-nascent polypeptide chain complex to the heterotrimeric Sec61 (composed of the ␣-, ␤-, and ␥ subunits) complex embedded in the membrane of the endoplasmic reticulum (ER) (for review, see Rapoport et al

A convenient procedure for the synthesis of 2-heterosubstituted statine derivatives as novel building blocks in HIV-protease inhibitors has been developed. The synthesis starts with protected L-phenylalaninols, which were converted to gamma-amino alpha, beta-unsaturated esters in a one-pot procedure. A highly diastereoselective epoxidation of the N-protected (E)-enoates, followed by regioselective ring opening of the corresponding 2,3-epoxy esters with a variety of heteronucleophiles, resulted in 2-heterosubstituted statine derivatives. The overall stereo-chemical outcome of the transformations meets the required configuration of HIV-protease inhibitors. The short, synthetically flexible, and highly diastereoselective synthesis of 2-heterosubstituted statines has enabled a broad derivation, covering the S3, S2, and S1'-S3' sites of the enzyme. In a series of 46 derivatives, several potent inhibitors were obtained with Ki values as low as 3.4 nM and antiviral activity in the lower nanomolar-range. The structural parameters of the compounds which determine the potency of inhibition and selectivity for the viral enzyme are discussed.

The initial step of influenza infection is binding of the virus particles via their hemagglutinin to cell-surface sialic acids. This study was initiated to elucidate the functional groups of the nine-carbon sialic acid molecule which interact with the hemagglutinin and contribute to the affinity of this sugar to the protein. In order to address this question, synthetic sialic acid analogues were tested in a virus adsorption inhibition assay for their inhibitory potency. Modifications in three regions of the sialic acid molecule were evaluated: the glycerol side chain (C7 -C9), the N-acetyl group at C5, and the carboxy group (Cl). In the glycerol side chain, the hydroxy groups at C7 and C8 appear to be important for binding through hydrogen bonds, whereas the hydroxyl at C9 does not appear to be involved. The N-acetyl group is critical for the interaction of sialic acid with the hemagglutinin. The results suggest that its contribution is mediated through hydrophobic interactions of the methyl group. Finally, the orientation of the carboxy group is essential for the binding of sialic acid to the hemagglutinin. The information gained in this study will be useful in developing novel compounds which bind more avidly to the influenza virus hemagglutinin. Such a strategy may contribute to the design of new anti-influenza drugs.

The reaction of peracetylated N-acetylneuraminic acid methyl ester 1 with nitrosyl acetate affords a mixture of antilsyn-Nnitroso derivatives 2a and 2a', which can be transformed with sodium trifluoroethanolate into the 5-diazo compound 3. The reaction of 3 with tetrabutylammonium acetate/acetic acid yields methyl 2,4,5,?,8,9-hexa-0-acety~-3-deoxy-~-cIycero-~-~-gQ~Qcto-2-nonu~opyranosidonate (KDN) (4a). On the other hand, 5-azido-5-deoxy-KDN (5a) is formed by treating 3 with hydrazoic acid in toluene. Heating of 2a/2a' in toluene furnishes a mixture of the peracetylated deaminated furanoid sialic acid derivative 10 with probably changed configuration at C-5 and C-6 and the deaminated 5,6-and 4,5-didehydrosialic acid derivatives 6 and 7. Catalytic hydrogenation of 6 and 7 yields the 5-deoxy-KDN derivative 9 a and -after hydrogenolysis of the 4-acetoxy group -a minor amount of compound 8. The compounds 4a, 5a, and 9 a have been converted into the free deaminated sialic acids 4e, 5e, and 9e via the 4-methylumbelliferyl 2a-glycosides 4c, d. Sc, d, and 9c, d.The deaminated neuraminic acid 3-deoxy-~-glycero-~-galacto-nonulosonic acid (KDN) has been first isolated by Inoue and co-workers2) from the membrane polysialoglycoproteins of Salmo gaidneri (rainbow trout) eggs. Its function as a terminal unit is obviously the protection of the membrane against bacterial sialidases2' because they are not able to release sialic acid residues without the carbonyl function of the Sacetamido group3). For instance, the 2-a-umbelliferyl glycosides of KDN, 5-deoxy-KDN and 5-azido-5-deoxy-KDN are not cleaved significantly by Vibrio cholerae sialidase3! In addition, it has been found that the corresponding 2,3-didehydrosialic acids are not inhibitors of this enzyme3). On the other hand, KDN can be activated by CMP-sialate synthase4). Therefore, it can be expected that this sialic acid analogue can be introduced into biologically interesting glycoproteins in order to protect them against sialidase activities of some bacteria. This paper describes the feasibility to synthesize KDN using N-acetylneuraminic acid as a starting material. Recently, a synthesis of KDN has been achieved by Auge and Gautheron5) by an enzymatic reaction of D-mannose with pyruvate. Furthermore, 5-azido-5-deoxy-KDN5' has been envisaged as an interesting substance for possible bioassays and as a substrate for studies of its behaviour towards the enzymes of the sialic acid metabolism. The synthesis of 5-deoxy-KDN') has also been considered a goal with respect to enzymatic studies.In Schemes 1,2, and 3 all steps leading to the desired title compounds are compiled. In the first step of Scheme 1 the peracetylated methyl ester 1 of Neu5Ac6) is treated with nitrosyl acetate7) to give a mixture of the two possible isomeric N-nitroso compounds 2a and 2a'. The nitroso group causes a characteristic downfield shift of the methyl signal of the acetamido group to