Timm Maier

Architecture of Mammalian Fatty Acid Synthase at 4.5 Å Resolution

The homodimeric mammalian fatty acid synthase is one of the most complex cellular multienzymes, in that each 270-kilodalton polypeptide chain carries all seven functional domains required for fatty acid synthesis. We have calculated a 4.5 angstrom–resolution x-ray crystallographic map of porcine fatty acid synthase, highly homologous to the human multienzyme, and placed homologous template structures of all individual catalytic domains responsible for the cyclic elongation of fatty acid chains into the electron density. The positioning of domains reveals the complex architecture of the multienzyme forming an intertwined dimer with two lateral semicircular reaction chambers, each containing a full set of catalytic domains required for fatty acid elongation. Large distances between active sites and conformational differences between the reaction chambers demonstrate that mobility of the acyl carrier protein and general flexibility of the multienzyme must accompany handover of the reaction intermediates during the reaction cycle.

Trigger Factor in Complex with the Ribosome Forms a Molecular Cradle for Nascent Proteins

During protein biosynthesis, nascent polypeptide chains that emerge from the ribosomal exit tunnel encounter ribosome-associated chaperones, which assist their folding to the native state. Here we present a 2.7 Å crystal structure of Escherichia coli trigger factor, the best-characterized chaperone of this type, together with the structure of its ribosome-binding domain in complex with the Haloarcula marismortui large ribosomal subunit. Trigger factor adopts a unique conformation resembling a crouching dragon with separated domains forming the amino-terminal ribosome-binding ‘tail’, the peptidyl-prolyl isomerase ‘head’, the carboxy-terminal ‘arms’ and connecting regions building up the ‘back’. From its attachment point on the ribosome, trigger factor projects the extended domains over the exit of the ribosomal tunnel, creating a protected folding space where nascent polypeptides may be shielded from proteases and aggregation. This study sheds new light on our understanding of co-translational protein folding, and suggests an unexpected mechanism of action for ribosome-associated chaperones.

The Crystal Structure of a Mammalian Fatty Acid Synthase

Mammalian fatty acid synthase is a large multienzyme that catalyzes all steps of fatty acid synthesis. We have determined its crystal structure at 3.2 angstrom resolution covering five catalytic domains, whereas the flexibly tethered terminal acyl carrier protein and thioesterase domains remain unresolved. The structure reveals a complex architecture of alternating linkers and enzymatic domains. Substrate shuttling is facilitated by flexible tethering of the acyl carrier protein domain and by the limited contact between the condensing and modifying portions of the multienzyme, which are mainly connected by linkers rather than direct interaction. The structure identifies two additional nonenzymatic domains: (i) a pseudo-ketoreductase and (ii) a peripheral pseudo-methyltransferase that is probably a remnant of an ancestral methyltransferase domain maintained in some related polyketide synthases. The structural comparison of mammalian fatty acid synthase with modular polyketide synthases shows how their segmental construction allows the variation of domain composition to achieve diverse product synthesis.

The Structure of a Cytolytic Alpha-Helical Toxin Pore Reveals its Assembly Mechanism

Pore-forming toxins (PFTs) are a class of potent virulence factors that convert from a soluble form to a membrane-integrated pore. They exhibit their toxic effect either by destruction of the membrane permeability barrier or by delivery of toxic components through the pores. Among the group of bacterial PFTs are some of the most dangerous toxins, such as diphtheria and anthrax toxin. Examples of eukaryotic PFTs are perforin and the membrane-attack complex, proteins of the immune system. PFTs can be subdivided into two classes, α-PFTs and β-PFTs, depending on the suspected mode of membrane integration, either by α-helical or β-sheet elements. The only high-resolution structure of a transmembrane PFT pore is available for a β-PFT—α-haemolysin from Staphylococcus aureus. Cytolysin A (ClyA, also known as HlyE), an α-PFT, is a cytolytic α-helical toxin responsible for the haemolytic phenotype of several Escherichia coli and Salmonella enterica strains. ClyA is cytotoxic towards cultured mammalian cells, induces apoptosis of macrophages and promotes tissue pervasion. Electron microscopic reconstructions demonstrated that the soluble monomer of ClyA must undergo large conformational changes to form the transmembrane pore. Here we report the 3.3 Å crystal structure of the 400 kDa dodecameric transmembrane pore formed by ClyA. The tertiary structure of ClyA protomers in the pore is substantially different from that in the soluble monomer. The conversion involves more than half of all residues. It results in large rearrangements, up to 140 Å, of parts of the monomer, reorganization of the hydrophobic core, and transitions of β-sheets and loop regions to α-helices. The large extent of interdependent conformational changes indicates a sequential mechanism for membrane insertion and pore formation.

Architecture of a Fungal Fatty Acid Synthase at 5 Å Resolution

All steps of fatty acid synthesis in fungi are catalyzed by the fatty acid synthase, which forms a 2.6-megadalton α6β6 complex. We have determined the molecular architecture of this multienzyme by fitting the structures of homologous enzymes that catalyze the individual steps of the reaction pathway into a 5 angstrom x-ray crystallographic electron density map. The huge assembly contains two separated reaction chambers, each equipped with three sets of active sites separated by distances up to ∼130 angstroms, across which acyl carrier protein shuttles substrates during the reaction cycle. Regions of the electron density arising from well-defined structural features outside the catalytic domains separate the two reaction chambers and serve as a matrix in which domains carrying the various active sites are embedded. The structure rationalizes the compartmentalization of fatty acid synthesis, and the spatial arrangement of the active sites has specific implications for our understanding of the reaction cycle mechanism and of the architecture of multienzymes in general.

The multienzyme architecture of eukaryotic fatty acid synthases.

Eukaryotic fatty acid synthases (FASs) are huge multifunctional enzymes that carry out all enzymatic steps essential for fatty acid biosynthesis. Recent crystallographic studies provide new insights into the architecture of the two distinct eukaryotic FAS systems, the 2.6 MDa heterododecameric fungal and the 540 kDa dimeric animal FAS. In this review, we compare the fundamentally different organization of these two megasynthases and discuss the structural principles of enzyme integration and substrate shuttling in FAS multienzymes.

Molecular mechanism and structure of Trigger Factor bound to the translating ribosome

Ribosome‐associated chaperone Trigger Factor (TF) initiates folding of newly synthesized proteins in bacteria. Here, we pinpoint by site‐specific crosslinking the sequence of molecular interactions of Escherichia coli TF and nascent chains during translation. Furthermore, we provide the first full‐length structure of TF associated with ribosome–nascent chain complexes by using cryo‐electron microscopy. In its active state, TF arches over the ribosomal exit tunnel accepting nascent chains in a protective void. The growing nascent chain initially follows a predefined path through the entire interior of TF in an unfolded conformation, and even after folding into a domain it remains accommodated inside the protective cavity of ribosome‐bound TF. The adaptability to accept nascent chains of different length and folding states may explain how TF is able to assist co‐translational folding of all kinds of nascent polypeptides during ongoing synthesis. Moreover, we suggest a model of how TFs chaperoning function can be coordinated with the co‐translational processing and membrane targeting of nascent polypeptides by other ribosome‐associated factors.

Structure of the Type VI Secretion System Contractile Sheath

Bacteria use rapid contraction of a long sheath of the type VI secretion system (T6SS) to deliver effectors into a target cell. Here, we present an atomic-resolution structure of a native contracted Vibrio cholerae sheath determined by cryo-electron microscopy. The sheath subunits, composed of tightly interacting proteins VipA and VipB, assemble into a six-start helix. The helix is stabilized by a core domain assembled from four β strands donated by one VipA and two VipB molecules. The fold of inner and middle layers is conserved between T6SS and phage sheaths. However, the structure of the outer layer is distinct and suggests a mechanism of interaction of the bacterial sheath with an accessory ATPase, ClpV, that facilitates multiple rounds of effector delivery. Our results provide a mechanistic insight into assembly of contractile nanomachines that bacteria and phages use to translocate macromolecules across membranes.

Architecture of Human Mtor Complex 1

From sensing leucine to metabolic control The mTORC1 protein kinase complex plays central roles in regulating cell growth and metabolism and is implicated in common human diseases such as diabetes and cancer. The level of the amino acid leucine tells an organism a lot about its physiological state, including how much food is available, how much insulin is going to be needed, and whether new muscle mass can be made (see the Perspective by Buel and Blenis). Wolfson et al. identified a biochemical sensor of leucine, Sestrin2, which connects the concentration of leucine to the control of organismal metabolism and growth. When leucine bound to Sestrin2, it was released from a complex with the mTORC1 regulatory factor GATOR2, activating the mTORC1 complex. Saxton et al. describe the crystal structure of Sestrin2 and show how it specifically detects leucine. Aylett et al. determined the structure of human mTORC1 by cryoelectron microscopy and the crystal structure of a regulatory subunit, Raptor. The results reveal the structural basis for the function and intricate regulation of this important enzyme, which is also a strategic drug target. Science, this issue p. 43, p. 48, p. 53; see also p. 25 Electron microscopy reveals the structure of a key regulator of growth and metabolism. [Also see Perspective by Buel and Blenis] Target of rapamycin (TOR), a conserved protein kinase and central controller of cell growth, functions in two structurally and functionally distinct complexes: TORC1 and TORC2. Dysregulation of mammalian TOR (mTOR) signaling is implicated in pathologies that include diabetes, cancer, and neurodegeneration. We resolved the architecture of human mTORC1 (mTOR with subunits Raptor and mLST8) bound to FK506 binding protein (FKBP)–rapamycin, by combining cryo–electron microscopy at 5.9 angstrom resolution with crystallographic studies of Chaetomium thermophilum Raptor at 4.3 angstrom resolution. The structure explains how FKBP-rapamycin and architectural elements of mTORC1 limit access to the recessed active site. Consistent with a role in substrate recognition and delivery, the conserved amino-terminal domain of Raptor is juxtaposed to the kinase active site.

The X-ray Crystal Structure of Human β-Hexosaminidase B Provides New Insights into Sandhoff Disease

Human lysosomal beta-hexosaminidases are dimeric enzymes composed of alpha and beta-chains, encoded by the genes HEXA and HEXB. They occur in three isoforms, the homodimeric hexosaminidases B (betabeta) and S (alphaalpha), and the heterodimeric hexosaminidase A (alphabeta), where dimerization is required for catalytic activity. Allelic variations in the HEXA and HEXB genes cause the fatal inborn errors of metabolism Tay-Sachs disease and Sandhoff disease, respectively. Here, we present the crystal structure of a complex of human beta-hexosaminidase B with a transition state analogue inhibitor at 2.3A resolution (pdb 1o7a). On the basis of this structure and previous studies on related enzymes, a retaining double-displacement mechanism for glycosyl hydrolysis by beta-hexosaminidase B is proposed. In the dimer structure, which is derived from an analysis of crystal packing, most of the mutations causing late-onset Sandhoff disease reside near the dimer interface and are proposed to interfere with correct dimer formation. The structure reported here is a valid template also for the dimeric structures of beta-hexosaminidase A and S.