Structural basis for human monoglyceride lipase inhibition

Monoglyceride lipase (MGL) is a serine hydrolase that hydrolyses 2-arachidonoylglycerol (2-AG) into arachidonic acid and glycerol. 2-AG is an endogenous ligand of cannabinoid receptors, involved in various physiological processes in the brain. We present here the first crystal structure of human MGL in its apo form and in complex with the covalent inhibitor SAR629. MGL shares the classic fold of the alpha/beta hydrolase family but depicts an unusually large hydrophobic occluded tunnel with a highly flexible lid at its entry and the catalytic triad buried at its end. Structures reveal the configuration of the catalytic triad and the shape and nature of the binding site of 2-AG. The bound structure of SAR629 highlights the key interactions for productive binding with MGL. The shape of the tunnel suggests a high druggability of the protein and provides an attractive template for drug discovery.

Expression, purification and crystallization of human phosphotyrosine phosphatase 1B

Protein phosphotyrosine phosphatases are believed to be involved in the regulation of the activity of cellular proteins, such as receptor tyrosine kinases, by controlling their phosphorylation status. One of the best described and characterized protein of this class of enzymes is the phosphotyrosine phosphatase 1B. To obtain sufficient quantities for structural investigations, truncated forms of PTP1B encompassing the catalytic domain were over-expressed in Escherichia coli and purified to apparent homogeneity by conventional chromatography. The activity of these purified enzymes has been compared with the wild-type enzyme expressed in mammalian cells. By measuring the activities against p-nitrophenyl phosphate, the pH dependence of this activity, and responses to different modulators, it could be demonstrated that the truncated forms of PTP1B retained the same characteristics as the full-length mammalian enzyme, but are not subject to inhibition of enzymic activity mediated by the C-terminus. Due to their improved solubility, it can be assumed that the catalytic domains are advantageous for crystallization studies in comparison to the natural enzyme. In a screening for crystallization conditions, we obtained protein crystals indicating that the quality of the purified protein is sufficient for crystallographic studies.

Modification of aminoacyl-tRNA synthetases with pyridoxal-5'-phosphate. Identification of the labeled amino acid residues

The isotopic [32P]PPi-ATP exchange activity of isoleucyl-, valyl-, histidyl-, tyrosyl- and methionyl-tRNA synthetases from Escherichia coli are lost upon incubation in the presence of pyridoxal-5'-phosphate (PLP). When the residual activity of either isoleucyl-, valyl- or methionyl-tRNA synthetase (monomeric truncated form) was plotted as a function of the number of PLP molecules incorporated per enzyme molecule, the plots obtained appeared biphasic. Below 50% inactivation of these enzymes, PLP incorporation varied linearly with the isotopic exchange measurements, and extrapolation of the first half of the plot indicated a stoichiometry of 1.10 +/- 0.05 mol of PLP incorporated per mol of 100% inactivated synthetase. Beyond 50% inactivation, the graph deviated from its initial slope, and up to 4-5 mol of PLP were incorporated per mol of synthetase at the highest used PLP concentrations. In the cases of homodimeric histidyl- and tyrosyl-tRNA synthetases, extrapolation of the graph at 100% inactivation indicated 2.8 +/- 0.1 and 2.4 +/- 0.1 mol of PLP incorporated per mol of enzyme, respectively. PLP-labeled peptides were obtained through trypsin digestion and RPLC purification, prior to Edman degradation analysis. PLP-labeled residues were identified as lysines 132, 332, 335 and 402 of monomeric methionyl-tRNA synthetase, lysines 332, 335, 402, 465, 596 and 640 of native dimeric methionyl-tRNA synthetase, lysines 22, 117, 601, 604 and 645 of isoleucyl-tRNA synthetase, lysines 554, 557, 559, 593 and 909 of valyl-tRNA synthetase, lysines 2, 118, 369 and 370 of histidyl-tRNA synthetase, and lysine 237 of tyrosyl-tRNA synthetase. In addition, the amino terminal residue of the polypeptide chain(s) of either isoleucyl-, valyl-, histidyl- or methionyl-tRNA synthetases was found labeled. Among these residues, lysines 332, 335 and 402 of monomeric methionyl-tRNA synthetase as well as lysines 332, 335, 402 and 596 of dimeric methionyl-tRNA synthetase, lysines 601, 604 and 645 of isoleucyl-tRNA synthetase, lysines 554, 557 and 559 of valyl-tRNA synthetase, lysines 2, 369 and 370 of histidyl-tRNA synthetase, and lysine 237 of tyrosyl-tRNA synthetase were labeled in the presence of PLP concentrations smaller than or equal to 1 mM, and are shown to be critical for the activity of the enzymes. It is concluded that these residues participate to the binding sites of the phosphates of ATP on the studied synthetases.

Peptide and protein carboxyl-terminal labeling through carboxypeptidase Y-catalyzed transpeptidation

A survey of carboxypeptidase Y-catalyzed carboxyl-terminal modification of short peptides in the presence of various amino acids revealed that transpeptidation occurred in significant yield only with peptides containing a proline at the penultimate or antepenultimate position. For these peptides, transpeptidation was shown to occur specifically at the carboxyl side of the proline, thus suggesting a determining role of this residue for transpeptidation. Two model peptides, YPFP-GPI and YPFVEPI, were studied in detail. Initial yields of transpeptidation in the presence of various nucleophiles were compared. Among natural amino acids, the highest yield was obtained with methionine, followed by other amino acids bearing hydrophobic side chains. In order to transpose the method of transpeptidation to a protein, a variant of Escherichia coli methionyl-tRNA synthetase bearing the carboxyl-terminal Glu-Pro-Met sequence was genetically created. Under the conditions optimized for the transpeptidation of YPF-VEPI with methionine, this protein could be labeled specifically at its carboxyl-terminal end. Moreover, the parameters of the labeling reaction were in agreement with those observed in the transpeptidation of the model peptide.