Cysteine cathepsins play an indispensable role in proteolytic processing of the major histocompatibility complex class II-associated invariant chain (Ii) and foreign antigens in a number of antigen presenting cells. Previously it was shown that a fragment of 64 residues present in the p41 form of the Ii (p41 fragment) selectively inhibits the endopeptidase cathepsin L, whereas the activity of cathepsin S remains unaffected. Comparison of structures indicated that the selectivity of interactions between cysteine cathepsins and the p41 fragment is far from being understood and requires further investigation. The p41 fragment has now been shown also to inhibit human cathepsins V, K, and F (also, presumably, O) and mouse cathepsin L with K i values in the low nanomolar range. These K i values are sufficiently low to ensure complex formation at physiological concentrations. In addition we have found that the p41 fragment can inhibit cathepsin S too. These findings suggest that regulation of the proteolytic activity of most of the cysteine cathepsins by the p41 fragment is an important and widespread control mechanism of antigen presentation.In the processes of adaptive immunity, antigenic proteins are cleaved to peptides, some of which are loaded into the binding cleft of MHC 2 class II molecules for presentation on the surface of professional antigen-presenting cells (APCs) (1, 2). The degradation of antigenic proteins takes place in the endocytic compartments (endosomes and lysosomes) rich in proteolytic activity. The current list of protein-degrading enzymes in these compartments includes the group of cysteine cathepsins, asparaginyl endoprotease, the aspartic proteases cathepsins D and E, and thiol reductase GILT (3-5). This work is focused on cysteine cathepsins. Studies on gene knock-out mice showed that the pattern of antigenic peptide generation is not affected by the absence of cathepsins L and S (6), indicating redundant and overlapping roles within the group of cysteine cathepsins, although in certain cases specific roles in the degradation of antigens have been assigned to particular proteases (7,8).It is thought that the activity of proteases in the endocytic mixture is also regulated by protein inhibitors (4). A number of different inhibitors of cysteine cathepsins, including cystatins C and F and stefins A and B, have so far been localized in different types of APCs (9); however, direct and precise evidence regarding their involvement in the regulation of endosomal protease activity in APCs is still lacking (10, 11). Even more intriguing and less well investigated is the role of the inhibitory fragment of the p41 form of Ii (p41 fragment).Ii associates with the ␣ and ␤ chains of MHC class II molecules in the endoplasmic reticulum and is responsible for their proper folding and trafficking to endosomes (12), where the MHC class II molecules are freed from Ii by proteolytic processing of Ii. Ii exists in mouse in two (and in human, four) alternately spliced forms, p31 and p41, distinguished by an additio...

SUMMARY Legumain (AEP) is a lysosomal cysteine protease that is a lysosomal cysteine protease that was first characterized in leguminous seeds and later discovered in higher eukaryotes. AEP up-regulation is linked to a number of diseases including inflammation, arteriosclerosis and tumorigenesis. Thus legumain is an excellent molecular target for the development of new chemical markers. We deployed a hybrid combinatorial substrate library (HyCoSuL) approach to obtain P1-Asp fluorogenic substrates and biotin-labeled inhibitors that targeted legumain. Since this approach led to probes that were also recognized by caspases, we introduced a Counter Selection Substrate Library (CoSeSuL) approach that biases the peptidic scaffold against caspases, thus delivering highly selective legumain probes. The selectivity of these tools was validated using M38L and HEK293 cells. We also propose that the CoSeSuL methodology can be considered as a general principle in the design of selective probes for other protease families where selectivity is difficult to achieve by conventional sequence-based profiling.

Mammalian cathepsin C is primarily responsible for the removal of N-terminal dipeptides and activation of several serine proteases in inflammatory or immune cells, while its malarial parasite ortholog dipeptidyl aminopeptidase 1 plays a crucial role in catabolizing the hemoglobin of its host erythrocyte. In this report, we describe the systematic substrate specificity analysis of three cathepsin C orthologs from Homo sapiens (human), Bos taurus (bovine) and Plasmodium falciparum (malaria parasite). Here, we present a new approach with a tailored fluorogenic substrate library designed and synthesized to probe the S1 and S2 pocket preferences of these enzymes with both natural and a broad range of unnatural amino acids. Our approach identified very efficiently hydrolyzed substrates containing unnatural amino acids, which resulted in the design of significantly better substrates than those previously known. Additionally, in this study significant differences in terms of the structures of optimal substrates for human and malarial orthologs are important from the therapeutic point of view. These data can be also used for the design of specific inhibitors or activity-based probes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00726-013-1654-2) contains supplementary material, which is available to authorized users.

Bacterial cell wall proteins play crucial roles in cell survival, growth, and environmental interactions. In Gram-positive bacteria, cell wall proteins include several types that are non-covalently attached via cell wall binding domains. Of the two conserved surface-layer (S-layer)-anchoring modules composed of three tandem SLH or CWB2 domains, the latter have so far eluded structural insight. The crystal structures of Cwp8 and Cwp6 reveal multi-domain proteins, each containing an embedded CWB2 module. It consists of a triangular trimer of Rossmann-fold CWB2 domains, a feature common to 29 cell wall proteins in Clostridium difficile 630. The structural basis of the intact module fold necessary for its binding to the cell wall is revealed. A comparison with previously reported atomic force microscopy data of S-layers suggests that C. difficile S-layers are complex oligomeric structures, likely composed of several different proteins.

Mycocypins, clitocypins and macrocypins, are cysteine protease inhibitors isolated from the mushrooms Clitocybe nebularis and Macrolepiota procera. Lack of sequence homology to other families of protease inhibitors suggested that mycocypins inhibit their target cysteine protease by a unique mechanism and that a novel fold may be found. The crystal structures of the complex of clitocypin with the papain-like cysteine protease cathepsin V and of macrocypin and clitocypin alone have revealed yet another motif of binding to papain like-cysteine proteases, which in a yet unrevealed way occludes the catalytic residue. The binding is associated with a peptide-bond flip of glycine that occurs before or concurrently with the inhibitor docking. Mycocypins possess a ␤-trefoil fold, the hallmark of Kunitz-type inhibitors. It is a tree-like structure with two loops in the root region, a stem comprising a six-stranded ␤-barrel, and two layers of loops (6 ؉ 3) in the crown region. The two loops that bind to cysteine cathepsins belong to the lower layer of the crown loops, whereas a single loop from the crown region can inhibit trypsin or asparaginyl endopeptidase, as demonstrated by site-directed mutagenesis. These loops present a versatile surface with the potential to bind to additional classes of proteases. When appropriately engineered, they could provide the basis for possible exploitation in crop protection.Inhibition of foreign protease activity is a widespread defense mechanism in plants against their pests, pathogens, and parasites (1). Protein inhibitors of proteases are present in a variety of plant tissues. They can be deployed alone or together with a variety of small molecules (2). It has been known for a long time that the expression of protease inhibitors is increased in injured plant leaves (3) and that their expression can be induced as a response to attack by insects or pathogens (2).Given the negative environmental effects of chemical pesticides used in crop protection, it is important to explore alternative approaches, such as the incorporation of genes encoding protease inhibitors into plants. Transgenic plants expressing various protease inhibitors have shown enhanced levels of insect resistance; however, the adaptive capacity of insect digestive proteases limits the use of single protease inhibitors (4, 5). The use of hybrid protease inhibitors with multiple inhibitory activity could, however, affect the functional properties of the fused inhibitors (6). Incorporation of genes encoding a range of protease inhibitors is to run the risk of deleterious modification of plants, but the use of a single protease inhibitor with versatile functionality could be the way forward.With these in mind, we have undertaken structural and mechanistic studies of the cysteine protease inhibitors clitocypin (Clt) 3 from mushroom Basidiomycetes Clitocybe nebularis and macrocypins (Mcp) from Macrolepiota procera. Based on the lack of sequence similarity to other protease inhibitors, they form separate protease inhibitor famil...