Investigation of the PDZ domain ligand binding site using chemically modified peptides

Several chemically modified analogues to a tightly binding ligand for the second PDZ domain of MAGI-3 were synthesized and evaluated for their ability to compete with native peptide ligands. N-methyl scanning of the ligand backbone amides revealed the energetically important hydrogen bonds between the ligand backbone and the PDZ domain. Analogues to the ligand's conserved threonine/serine(-2) residue, involved in a side chain to side chain hydrogen bond with a conserved histidine in the PDZ domain, revealed that the interaction is highly sensitive to the steric structure around the hydroxyl group of this residue. Analogues of the ligand carboxy terminus revealed that the full hydrogen bond network of the GLGF loop is important in ligand binding.

Copper-mediated cross-coupling of aryl boronic acids and alkyl thiols

[reaction: see text] The cross-coupling of aryl boronic acids and alkanethiols mediated by copper(II) acetate and pyridine in anhydrous dimethylformamide affords aryl alkyl sulfides in good yield with a wide variety of substituted aryl boronic acids. The method is applicable to the synthesis of aryl sulfides of cysteine.

Inhibition of sonic hedgehog autoprocessing in cultured mammalian cells by sterol deprivation

Sonic hedgehog (Shh) is a signaling molecule that is important for defining patterning in the developing vertebrate central nervous system. After translation, Shh autoproteolyzes and covalently attaches cholesterol to the newly formed carboxyl terminus, a modification crucial for normal Shh signaling. Presented here is evidence that acute severe sterol deprivation in cultured Chinese hamster ovary cells expressing mouse Shh (mShh) inhibits autoprocessing of the protein. These conditions allowed the first detailed kinetic analysis of mShh autoprocessing and turnover rates revealing that cells rapidly degrade both precursor and mature mShh regardless of sterol content and sterol deprivation increases the rate of precursor degradation. Inhibition of mShh autoprocessing also allowed the determination of the subcellular localization of mShh precursor which accumulates in a pre-medial Golgi intracellular compartment. Finally, the precursor form of mShh that results from autoprocessing inhibition appears to accumulate as an amide rather than a stable thioester.

MAGI-1, a membrane-associated guanylate kinase with a unique arrangement of protein-protein interaction domains

Membrane-associated guanylate kinase (MAGUK) proteins participate in the assembly of multiprotein complexes on the inner surface of the plasma membrane at regions of cell-cell contact. MAGUKs are characterized by three types of protein-protein interaction modules: the PDZ domain, the Src homology 3 (SH3) domain, and the guanylate kinase (GuK) domain. The arrangement of these domains is conserved in all previously known MAGUKs: either one or three PDZ domains in the NH2-terminal half, followed by the SH3 domain, followed by a COOH-terminal GuK domain. In this report, we describe the cDNA cloning and subcellular distribution of MAGI-1, a MAGUK with three unique structural features: 1) the GuK domain is at the NH2 terminus, 2) the SH3 domain is replaced by two WW domains, and 3) it contains five PDZ domains. MAGI-1 mRNA was detected in several adult mouse tissues. Sequence analysis of overlapping cDNAs revealed the existence of three splice variants that are predicted to encode MAGI-1 proteins with different COOH termini. The longest variant, MAGI-1c, contains three bipartite nuclear localization signals in its unique COOH-terminal sequence and was found predominantly in the nucleus of Madin-Darby canine kidney cells. A shorter form lacking these signals was found primarily in membrane and cytoplasmic fractions. This distribution, which is reminiscent of that seen for the tight junction protein ZO-1, suggests that MAGI-1 may participate in the transmission of regulatory signals from the cell surface to the nucleus.

Conformation of a water-soluble derivative of taxol in water by 2D-NMR spectroscopy

BACKGROUND

Taxol is a natural product produced by the Pacific yew, Taxus brevifolia, that has emerged as a prominent chemotherapeutic agent for the treatment of solid tumors. It binds to microtubules, stabilizing them and arresting cells in mitosis. Taxol has been produced synthetically and a wealth of structure-activity data has recently emerged. To data, however, no single conformational model exists for the interpretation of these data. Studies of taxol and its analogs in organic solvents showed two distinct conformations, one in which the 3'-benzamido group and the 2-benzoyl group are in close proximity, and another in which the 2-benzoyl group is instead close to the 3'-phenyl group. We decided to use a derivative of taxol that has improved water-solubility to determine the structure of taxol in water.

RESULTS

We have synthesized and characterized a stable water-soluble derivative of taxol that binds to microtubules and has a cytotoxicity profile very similar to that of taxol. 1D and 2D 1H NMR experiments with this bioactive compound in D2O indicate the presence of one conformer with a well-defined structure. In this structure, the 2-benzoyl group is clustered with the 3'-phenyl group.

CONCLUSION

The determination of the conformation of taxol in water may allow quantitative three-dimensional interpretation of the structure-activity data obtained for taxol, and hence enable the design of novel taxol mimics.

Total synthesis of taxol

Taxol, a substance originally isolated from the Pacific yew tree (Taxus brevifolia) more than two decades ago, has recently been approved for the clinical treatment of cancer patients. Hailed as having provided one of the most significant advances in cancer therapy, this molecule exerts its anticancer activity by inhibiting mitosis through enhancement of the polymerization of tubulin and consequent stabilization of microtubules. The scarcity of taxol and the ecological impact of harvesting it have prompted extension searches for alternative sources including semisynthesis, cellular culture production and chemical synthesis. The latter has been attempted for almost two decades, but these attempts have been thwarted by the magnitude of the synthetic challenge. Here we report the total synthesis of taxol by a convergent strategy, which opens a chemical pathway for the production of both the natural product itself and a variety of designed taxoids.