A novel coronavirus has been identified as the causative agent of severe acute respiratory syndrome (SARS). The viral main proteinase (M ^pro , also called 3CL ^pro ), which controls the activities of the coronavirus replication complex, is an attractive target for therapy. We determined crystal structures for human coronavirus (strain 229E) M ^pro and for an inhibitor complex of porcine coronavirus [transmissible gastroenteritis virus (TGEV)] M ^pro , and we constructed a homology model for SARS coronavirus (SARS-CoV) M ^pro . The structures reveal a remarkable degree of conservation of the substrate-binding sites, which is further supported by recombinant SARS-CoV M ^pro -mediated cleavage of a TGEV M ^pro substrate. Molecular modeling suggests that available rhinovirus 3C ^pro inhibitors may be modified to make them useful for treating SARS.

The key enzyme in coronavirus polyprotein processing is the viral main proteinase, M^pro, a protein with extremely low sequence similarity to other viral and cellular proteinases. Here, the crystal structure of the 33.1 kDa transmissible gastroenteritis (corona)virus M^pro is reported. The structure was refined to 1.96 Å resolution and revealed three dimers in the asymmetric unit. The mutual arrangement of the protomers in each of the dimers suggests that M^pro self‐processing occurs in trans. The active site, comprised of Cys144 and His41, is part of a chymotrypsin‐like fold that is connected by a 16 residue loop to an extra domain featuring a novel α‐helical fold. Molecular modelling and mutagenesis data implicate the loop in substrate binding and elucidate S1 and S2 subsites suitable to accommodate the side chains of the P1 glutamine and P2 leucine residues of M^pro substrates. Interactions involving the N‐terminus and the α‐helical domain stabilize the loop in the orientation required for trans‐cleavage activity. The study illustrates that RNA viruses have evolved unprecedented variations of the classical chymotrypsin fold.

Fungal cell walls play dynamic functions in interaction of fungi with their surroundings. In pathogenic fungi, the cell wall is the first structure to make physical contact with host cells. An important structural component of fungal cell walls is chitin, a well-known elicitor of immune responses in plants. Research into chitin perception has sparked since the chitin receptor from rice was cloned nearly a decade ago. Considering the widespread nature of chitin perception in plants, pathogens evidently evolved strategies to overcome detection, including alterations in the composition of cell walls, modification of their carbohydrate chains and secretion of effectors to provide cell wall protection or target host immune responses. Also non-pathogenic fungi contain chitin in their cell walls and are recipients of immune responses. Intriguingly, various mutualists employ chitin-derived signaling molecules to prepare their hosts for the mutualistic relationship. Research on the various types of interactions has revealed different molecular components that play crucial roles and, moreover, that various chitin-binding proteins contain dissimilar chitin-binding domains across species that differ in affinity and specificity. Considering the various strategies from microbes and hosts focused on chitin recognition, it is evident that this carbohydrate plays a central role in plant–fungus interactions.

While host immune receptors detect pathogen-associated molecular patterns to activate immunity, pathogens attempt to deregulate host immunity through secreted effectors. Fungi employ LysM effectors to prevent recognition of cell wall-derived chitin by host immune receptors, although the mechanism to compete for chitin binding remained unclear. Structural analysis of the LysM effector Ecp6 of the fungal tomato pathogen Cladosporium fulvum reveals a novel mechanism for chitin binding, mediated by intrachain LysM dimerization, leading to a chitin-binding groove that is deeply buried in the effector protein. This composite binding site involves two of the three LysMs of Ecp6 and mediates chitin binding with ultra-high (pM) affinity. Intriguingly, the remaining singular LysM domain of Ecp6 binds chitin with low micromolar affinity but can nevertheless still perturb chitin-triggered immunity. Conceivably, the perturbation by this LysM domain is not established through chitin sequestration but possibly through interference with the host immune receptor complex.

Membrane‐bound glutamate carboxypeptidase II (GCPII) is a zinc metalloenzyme that catalyzes the hydrolysis of the neurotransmitter N ‐acetyl‐l‐aspartyl‐l‐glutamate (NAAG) to N ‐acetyl‐l‐aspartate and l‐glutamate (which is itself a neurotransmitter). Potent and selective GCPII inhibitors have been shown to decrease brain glutamate and provide neuroprotection in preclinical models of stroke, amyotrophic lateral sclerosis, and neuropathic pain. Here, we report crystal structures of the extracellular part of GCPII in complex with both potent and weak inhibitors and with glutamate, the product of the enzyme's hydrolysis reaction, at 2.0, 2.4, and 2.2 Å resolution, respectively. GCPII folds into three domains: protease‐like, apical, and C‐terminal. All three participate in substrate binding, with two of them directly involved in C‐terminal glutamate recognition. One of the carbohydrate moieties of the enzyme is essential for homodimer formation of GCPII. The three‐dimensional structures presented here reveal an induced‐fit substrate‐binding mode of this key enzyme and provide essential information for the design of GCPII inhibitors useful in the treatment of neuronal diseases and prostate cancer.