Rhomboid proteases are evolutionary conserved intramembrane serine proteases. Because of their emerging role in many important biological pathways, rhomboids are potential drug targets. Unfortunately, few chemical tools are available for their study. Here, we describe a mass spectrometry-based assay to measure rhomboid substrate cleavage and inhibition. We have identified isocoumarin inhibitors and developed activity-based probes for rhomboid proteases. The probes can distinguish between active and inactive rhomboids due to covalent, reversible binding of the active-site serine and stable modification of a histidine residue. Finally, the structure of an isocoumarin-based inhibitor with Escherichia coli rhomboid GlpG uncovers an unusual mode of binding at the active site and suggests that the interactions between the 3-substituent on the isocoumarin inhibitor and hydrophobic residues on the protease reflect S′ subsite binding. Overall, these probes represent valuable tools for rhomboid study, and the structural insights may facilitate future inhibitor design.MALDI screening | covalent inhibition | regulated intramembrane proteolysis P roteolysis controls many important biological processes, such as apoptosis, antigen presentation, and blood coagulation. Selective digestion of protein substrates is possible by a combination of tight posttranslational control of protease activity (1) and the protease's substrate specificity, which generally is governed by the primary sequence around the scissile bond (2). The use of inhibitors and activity-based probes (ABPs) has led to a tremendous gain in understanding the roles of proteases within physiological and pathological processes (3). ABPs are small molecules that bind only to active enzymes, but not to zymogen or inhibitor-bound forms (4). ABPs generally consist of a detection tag, a spacer, and a "warhead." The warhead covalently binds to the target enzyme(s) and often is derived from a mechanism-based inhibitor. In the past, ABPs were used to study the activation, localization, and function of soluble proteases in a variety of organisms and disease models (5).Most proteases are soluble and surrounded by an aqueous environment. However, several families of intramembrane proteases exist (6-8): the metalloprotease family M50 (site-2 protease), the aspartic protease family A22 (signal peptide peptidase and γ-secretase), and the serine protease family S54 [rhomboid; numbering according to the MEROPS database (9)]. Rhomboid was discovered in 2001 as a protease in the EGF receptor signaling pathway in the fruitfly Drosophila melanogaster (10). Interestingly, rhomboid genes occur in all kingdoms of nature and are found in most sequenced organisms (11,12). Rhomboids appear to have a wide range of physiological functions, including bacterial protein export (13) and invasion by apicomplexan parasites (14,15), but the roles of many rhomboids remain to be discovered.Rhomboids catalyze peptide bond hydrolysis using a catalytic dyad formed by a serine residue in transmembrane domain...

Anabaena sp. PCC 7120 is a filamentous cyanobacterium that serves as a model to analyze prokaryotic cell differentiation, evolutionary development of plastids, and the regulation of nitrogen fixation. The cell wall is the cellular structure in contact with the surrounding medium. To understand the dynamics of the cell wall proteome during cell differentiation, the cell wall from Anabaena heterocysts was enriched and analyzed. In line with the recently proposed continuity of the outer membrane along the Anabaena filament, most of the proteins identified in the heterocyst cell-wall fraction are also present in the cell wall of vegetative cells, even though the lipid content of both membranes is different.

Impaired immune function contributes to the development of chronic obstructive pulmonary disease (COPD). Disease progression is further exacerbated by pathogen infections due to impaired immune responses. Elimination of infected cells is achieved by cytotoxic CD8+  T cells that are activated by MHC I-mediated presentation of pathogen-derived antigenic peptides. The immunoproteasome, a specialized form of the proteasome, improves generation of antigenic peptides for MHC I presentation thereby facilitating anti-viral immune responses. However, immunoproteasome function in the lung has not been investigated in detail yet. In this study, we comprehensively characterized the function of immunoproteasomes in the human and murine lung. Parenchymal cells of the lung express low constitutive levels of immunoproteasomes, while they are highly and specifically expressed in alveolar macrophages. Immunoproteasome expression is not altered in whole lung tissue of COPD patients. Novel activity-based probes and native gel analysis revealed that immunoproteasome activities are specifically and rapidly induced by IFNγ treatment in respiratory cells in vitro and by virus infection of the lung in mice. Our results suggest that the lung is potentially capable of mounting an immunoproteasome-mediated efficient adaptive immune response to intracellular infections.