Major histocompatibility class (MHC) II molecules are essential for running adaptive immune response. They are produced in the ER and targeted to late endosomes with the help of invariant chain (Ii) trimers. Ii trimerization may be induced by the Ii TM domain. To enable mechanistic and structural studies of MHC class II–Ii assembly, soluble forms of the complexes were expressed. We show that Ii trimerizes in the absence of the transmembrane part, prior to binding of a/b chains. The biochemical analysis supports the suggestion that the MHC class II–Ii complexes are not neces- sarily trimers of trimers, but that the Ii trimer can also be occupied by one or two MHC class II complexes.
COBISS.SI-ID: 26032935
Mushrooms are a rich source of novel proteins with unique features. Among them is cospin. It is a representative of one type of fungal protein-mediated defense against fungivorous insects. Cospin is a trypsin-specific protease inhibitor. It exhibits toxicity against the fruit fly. Cospin, the first fungal trypsin inhibitor with determined three-dimensional structure, utilizes a different loop for trypsin inhibition compared with other β-trefoil inhibitors.
COBISS.SI-ID: 25428775
Lectins are carbohydrate-binding proteins that exert their activity by binding to specific glycoreceptors. We here describe the crystal structures of rCNL in complex with lactose and LacdiNAc, defining its interactions with the sugars. CNL is a homodimeric lectin, each of whose monomers consist of a single ricin B lectin domain with its β-trefoil fold and one carbohydrate-binding site. Clitocybe nebularis lectin (CNL) showed biological activity, although its nonsugar-binding and monovalent mutants were inactive. This lead to the conclusion that the bivalent carbohydrate-binding property of CNL is essential for its activity. This suggests that understanding the interactions of lectins with glycans and elucidating their modes of action are necessary for their application in biomedicine.
COBISS.SI-ID: 25580583
It is more than 50 years since the lysosome was discovered. Since then its hydrolytic machinery, including proteases and other hydrolases, has been fairly well identified and characterized. Among these are the cyste- ine cathepsins, members of the family of papain-like cysteine proteases. They have unique reactive-site properties and an uneven tissue-specific expression pattern. In living organisms their activity is a delicate balance of expression, targeting, zymogen activation, inhibition by protein inhibitors and degradation. The specificity of their substrate binding sites, small-molecule inhibitor repertoire and crystal structures are providing new tools for research and development. Their unique reactive-site properties have made it possible to confine the targets simply by the use of appropriate reactive groups. The epoxysuccinyls still dominate the field, but now nitriles seem to be the most appropriate “warhead”. The view of cysteine cathepsins as lysosomal proteases is changing as there is now clear evidence of their localization in other cellular compartments. Besides being involved in protein turnover, they build an important part of the endosomal antigen presentation. Together with the growing number of non-endosomal roles of cysteine cathepsins is growing also the knowledge of their involvement in diseases such as cancer and rheumatoid arthritis, among others. Finally, cysteine cathepsins are important regulators and signaling molecules of an unimaginable number of biological processes. The current challenge is to identify their endogenous substrates, in order to gain an insight into the mechanisms of substrate degradation and processing. In this review, some of the remarkable advances that have taken place in the past decade are presented.
COBISS.SI-ID: 25347623
Protein protease inhibitors are the tools of nature in controlling proteolytic enzymes. They come in different shapes and sizes. The β-trefoil protease inhibitors that come from plants, first discovered by Kunitz, were later complemented with representatives from higher fungi. They inhibit serine (families S1 and S8) and cysteine proteases (families C1 and C13) as well as other hydrolases. Their versatility is the result of the plasticity of the loops coming out of the stable β-trefoil scaffold. For this reason, they display several different mechanisms of inhibition involving different positions of the loops and their combinations. Natural diversity, as well as the initial successes in de novo protein engineering, makes the β-trefoil proteins a promising starting point for the generation of strong, specific, multitarget inhibitors capable of inhibiting multiple types of hydrolytic enzymes and simultaneously interacting with different protein, carbohydrate, or DNA molecules. This pool of knowledge opens up new possibilities for the exploration of their naturally occurring as well as modified properties for applications in many fields of medicine, biotechnology, and agriculture.
COBISS.SI-ID: 26303015