Photo-crosslinking of proteins in intact cells reveals a dimeric structure of cyclooxygenase-2 and an inhibitor-sensitive oligomeric structure of microsomal prostaglandin E2 synthase-1

Structural Investigation of Proteins and Protein Complexes by Chemical Cross-Linking/Mass Spectrometry

During the last two decades, cross-linking combined with mass spectrometry (MS) has evolved as a valuable tool to gain structural insights into proteins and protein assemblies. Structural information is obtained by introducing covalent connections between amino acids that are in spatial proximity in proteins and protein complexes. The distance constraints imposed by the cross-linking reagent provide information on the three-dimensional arrangement of the covalently connected amino acid residues and serve as basis for de-novo or homology modeling approaches. As cross-linking/MS allows investigating protein 3D-structures and protein-protein interactions not only in-vitro, but also in-vivo, it is especially appealing for studying protein systems in their native environment. In this chapter, we describe the principles of cross-linking/MS and illustrate its value for investigating protein 3D-structures and for unraveling protein interaction networks.

Extending the cross-linking/mass spectrometry strategy: Facile incorporation of photo-activatable amino acids into the model protein calmodulin in Escherichia coli cells

Photo-induced cross-linking is a highly promising technique to investigate protein conformations and protein-protein interactions in their natural cellular environment. One strategy relies on the non-directed incorporation of diazirine-containing photo-activatable amino acids into proteins and a subsequent cross-link formation induced by UV-A irradiation. The advantage of this photo-cross-linking strategy is that it is not restricted to lysine residues and that hydrophobic regions in proteins can also be targeted, which is advantageous for investigating membrane proteins. Here, we present a simplified protocol that relies on the use of mineral salts medium without any special requirements for the incorporation of photo-methionines into proteins in Escherichia coli cells. The possibility to perform these experiments in E. coli is especially valuable as it is the major system for recombinant protein production. The method is exemplified for the Ca(2+) regulating protein calmodulin containing nine methionines, which were found to be replaced by their photo-activatable analogues. Our protocol allows the facile and stochastic incorporation of photo-methionines as the basis for conducting photo-cross-linking experiments in E. coli in an efficient manner.

Microsomal Prostaglandin E2 Synthase-1

The prostanoids and leukotrienes (LTs) formed from arachidonic acid (AA) via the cyclooxygenase (COX)-1/2 and 5-lipoxygenase (5-LO) pathway, respectively, mediate inflammatory responses, chronic tissue remodelling, cancer, asthma and autoimmune disorders, but also possess homeostatic functions in the gastrointestinal tract, uterus, brain, kidney, vasculature and host defence. Based on the manifold functions of these eicosanoids, the clinical use of non-steroidal anti-inflammatory drugs (NSAIDs), a class of drugs that block formation of all prostanoids, is hampered by severe side-effects including gastrointestinal injury, renal irritations and cardiovascular risks. Therefore, anti-inflammatory agents interfering with eicosanoid biosynthesis require a well-balanced pharmacological profile to minimize these on-target side-effects. Current anti-inflammatory research aims at identifying compounds that can suppress the massive formation of pro-inflammatory prostaglandin (PG)E2 without affecting homeostatic PGE2 and PGI2 synthesis. The inducible microsomal prostaglandin E2 synthase-1 (mPGES-1) is one promising target enzyme. We will give an overview about the structure, regulation and function of mPGES-1 and then present novel inhibitors of mPGES-1 that may possess a promising pharmacological profile.

Identification and development of mPGES-1 inhibitors: where we are at?

Microsomal prostaglandin E synthase-1 (mPGES-1) is the terminal synthase responsible for the synthesis of the pro-tumorigenic prostaglandin E(2) (PGE(2)). mPGES-1 is overexpressed in a wide variety of cancers. Since its discovery in 1997 by Bengt Samuelsson and collaborators, the enzyme has been the object of over 200 peer-reviewed articles. Although today mPGES-1 is considered a validated and promising therapeutic target for anticancer drug discovery, challenges in inhibitor design and selectivity are such that up to this date there are only a few published records of small-molecule inhibitors targeting the enzyme and exhibiting some in vivo anticancer activity. This review summarizes the structures, and the in vitro and in vivo activities of these novel mPGES-1 inhibitors. Challenges that have been encountered are also discussed.