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Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains-the 7-transmembrane domains-in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling.

Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains-the 7-transmembrane domains-in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling.

Polyketide natural products constitute a broad class of compounds with diverse structural features and biological activities. Their biosynthetic machinery, represented by type I polyketide synthases (PKSs), has an architecture in which successive modules catalyse two-carbon linear extensions and keto-group processing reactions on intermediates covalently tethered to carrier domains. Here we used electron cryo-microscopy to determine sub-nanometre-resolution three-dimensional reconstructions of a full-length PKS module from the bacterium Streptomyces venezuelae that revealed an unexpectedly different architecture compared to the homologous dimeric mammalian fatty acid synthase. A single reaction chamber provides access to all catalytic sites for the intramodule carrier domain. In contrast, the carrier from the preceding module uses a separate entrance outside the reaction chamber to deliver the upstream polyketide intermediate for subsequent extension and modification. This study reveals for the first time, to our knowledge, the structural basis for both intramodule and intermodule substrate transfer in polyketide synthases, and establishes a new model for molecular dissection of these multifunctional enzyme systems. Polyketide synthases are multidomain enzymes that produce polyketides, which form the basis of many therapeutic agents; here, electron cryo-microscopy is used to establish the structure of a bacterial full-length module, and to elucidate the structural basis of both intramodule and intermodule substrate transfer. Polyketide synthases (PKSs) are multi-domain enzyme complexes producing polyketides, a large class of secondary metabolites — in other words, natural products. Two papers from Georgios Skiniotis and colleagues use cryo-electron microscopy to probe the structure of an intact module of a full-length multienzyme PKS module involved in pikromycin biosynthesis in the bacterium Streptomyces venezuelae in different functional states. The structures reveal how the ketosynthase, acyltransferase, ketoreductase and acyl carrier protein (ACP) domains interact during the catalytic cycle. In each state the ACP is differentially positioned to facilitate intermediate transfer for the next catalytic step and for transfer to the next module.

Polyketide synthases (PKSs) are multidomain enzymes that produce polyketides, which form the basis of many therapeutic agents; here, electron cryo-microscopy is used to probe the structure of an intact module of a multi-enzyme PKS in different functional states. Polyketide synthases (PKSs) are multi-domain enzyme complexes producing polyketides, a large class of secondary metabolites — in other words, natural products. Two papers from Georgios Skiniotis and colleagues use cryo-electron microscopy to probe the structure of an intact module of a full-length multienzyme PKS module involved in pikromycin biosynthesis in the bacterium Streptomyces venezuelae in different functional states. The structures reveal how the ketosynthase, acyltransferase, ketoreductase and acyl carrier protein (ACP) domains interact during the catalytic cycle. In each state the ACP is differentially positioned to facilitate intermediate transfer for the next catalytic step and for transfer to the next module. The polyketide synthase (PKS) mega-enzyme assembly line uses a modular architecture to synthesize diverse and bioactive natural products that often constitute the core structures or complete chemical entities for many clinically approved therapeutic agents1. The architecture of a full-length PKS module from the pikromycin pathway of Streptomyces venezuelae creates a reaction chamber for the intramodule acyl carrier protein (ACP) domain that carries building blocks and intermediates between acyltransferase, ketosynthase and ketoreductase active sites (see accompanying paper2). Here we determine electron cryo-microscopy structures of a full-length pikromycin PKS module in three key biochemical states of its catalytic cycle. Each biochemical state was confirmed by bottom-up liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry. The ACP domain is differentially and precisely positioned after polyketide chain substrate loading on the active site of the ketosynthase, after extension to the β-keto intermediate, and after β-hydroxy product generation. The structures reveal the ACP dynamics for sequential interactions with catalytic domains within the reaction chamber, and for transferring the elongated and processed polyketide substrate to the next module in the PKS pathway. During the enzymatic cycle the ketoreductase domain undergoes dramatic conformational rearrangements that enable optimal positioning for reductive processing of the ACP-bound polyketide chain elongation intermediate. These findings have crucial implications for the design of functional PKS modules, and for the engineering of pathways to generate pharmacologically relevant molecules.

The surname of author Toon Laeremans was misspelled ‘Laermans’. This error has been corrected online.