Architecture of the human mTORC2 core complex.

The mammalian target of rapamycin (mTOR) is a key protein kinase controlling cellular metabolism and growth. It is part of the two structurally and functionally distinct multiprotein complexes mTORC1 and mTORC2. Dysregulation of mTOR occurs in diabetes, cancer and neurological disease. We report the architecture of human mTORC2 at intermediate resolution, revealing a conserved binding site for accessory proteins on mTOR and explaining the structural basis for the rapamycin insensitivity of the complex.

Architecture of human mTOR complex 1.

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.

STIL binding to Polo-box 3 of PLK4 regulates centriole duplication.

Polo-like kinases (PLK) are eukaryotic regulators of cell cycle progression, mitosis and cytokinesis; PLK4 is a master regulator of centriole duplication. Here, we demonstrate that the SCL/TAL1 interrupting locus (STIL) protein interacts via its coiled-coil region (STIL-CC) with PLK4 in vivo. STIL-CC is the first identified interaction partner of Polo-box 3 (PB3) of PLK4 and also uses a secondary interaction site in the PLK4 L1 region. Structure determination of free PLK4-PB3 and its STIL-CC complex via NMR and crystallography reveals a novel mode of Polo-box-peptide interaction mimicking coiled-coil formation. In vivo analysis of structure-guided STIL mutants reveals distinct binding modes to PLK4-PB3 and L1, as well as interplay of STIL oligomerization with PLK4 binding. We suggest that the STIL-CC/PLK4 interaction mediates PLK4 activation as well as stabilization of centriolar PLK4 and plays a key role in centriole duplication.

Co-crystallization with conformation-specific designed ankyrin repeat proteins explains the conformational flexibility of BCL-W.

BCL-W is a member of the BCL-2 family of anti-apoptotic proteins. A key event in the regulation of apoptosis is the heterodimerization between anti-apoptotic and pro-apoptotic family members, which involves a conserved surface-exposed groove on the anti-apoptotic proteins. Crystal structures of the ligand binding-competent conformation exist for all anti-apoptotic family members, with the exception of BCL-W, due to the flexibility of the BCL-W groove region. Existing structures had suggested major deviations of the BCL-W groove region from the otherwise structurally highly related remaining anti-apoptotic family members. To capture its ligand binding-competent conformation by counteracting the conformational flexibility of the BCL-W groove, we had selected high-affinity groove-binding designed ankyrin repeat proteins (DARPins) using ribosome display. We now determined two high-resolution crystal structures of human BCL-W in complex with different DARPins at resolutions 1.5 and 1.85Å, in which the structure of BCL-W is virtually identical, and BCL-W adopts a conformation extremely similar to the ligand-free conformation of its closest relative BCL-XL in both structures. However, distinct differences to all previous BCL-W structures are evident, notably in the ligand-binding region. We provide the first structural explanation for the conformational flexibility of the BCL-W groove region in comparison to other BCL-2 family members. Due to the importance of the anti-apoptotic BCL-2 family as drug targets, the presented crystal structure of ligand binding-competent BCL-W may serve as a valuable basis for structure-based drug design in the future and provides a missing piece for the structural characterization of this protein family.

Conserved sequence motifs and the structure of the mTOR kinase domain.

The atypical serine/threonine kinase mTOR (mammalian target of rapamycin) is a central regulator of cell growth and metabolism. mTOR is part of two multisubunit signalling complexes, mTORC1 and mTORC2. Although many aspects of mTOR signalling are understood, the lack of high-resolution structures impairs a detailed understanding of complex assembly, function and regulation. The structure of the kinase domain is of special interest for the development of mTOR inhibitors as anti-cancer agents. A homology model of the mTOR kinase domain was derived from the structure of PI3Ks (phosphoinositide 3-kinases). More recently, the crystal structure of the catalytic domain of human mTOR was determined, providing long-awaited structural insight into the architecture of mTOR. Interestingly, the homology model predicted several aspects of the crystal structure. In the present paper, we revisit the homology model in the context of the now available crystal structure of the mTOR kinase domain.

Structure and RNA-binding properties of the bacterial LSm protein Hfq.

Over the past years, small non-coding RNAs (sRNAs) emerged as important modulators of gene expression in bacteria. Guided by partial sequence complementarity, these sRNAs interact with target mRNAs and eventually affect transcript stability and translation. The physiological function of sRNAs depends on the protein Hfq, which binds sRNAs in the cell and promotes the interaction with their mRNA targets. This important physiological function of Hfq as a central hub of sRNA-mediated regulation made it one of the most intensely studied proteins in bacteria. Recently, a new model for sRNA binding by Hfq has been proposed that involves the direct recognition of the sRNA 3' end and interactions of the sRNA body with the lateral RNA-binding surface of Hfq. This review summarizes the current understanding of the RNA binding properties of Hfq and its (s)RNA complexes. Moreover, the implications of the new binding model for sRNA-mediated regulation are discussed.

Small RNA binding to the lateral surface of Hfq hexamers and structural rearrangements upon mRNA target recognition.

The bacterial Sm-like protein Hfq is a central player in the control of bacterial gene expression. Hfq forms complexes with small regulatory RNAs (sRNAs) that use complementary "seed" sequences to target specific mRNAs. Hfq forms hexameric rings, which preferably bind uridine-rich RNA 3' ends on their proximal surface and adenine-rich sequences on their distal surface. However, many reported properties of Hfq/sRNA complexes could not be explained by these RNA binding modes. Here, we use the RybB sRNA to identify the lateral surface of Hfq as a third, independent RNA binding surface. A systematic mutational analysis and competition experiments demonstrate that the lateral sites have a preference for and are sufficient to bind the sRNA "body," including the seed sequence. Furthermore, we detect significant structural rearrangements of the Hfq/sRNA complex upon mRNA target recognition that lead to a release of the seed sequence, or of the entire sRNA molecule in case of an unfavorable 3' end. Consequently, we propose a molecular model for the Hfq/sRNA complex, where the sRNA 3' end is anchored in the proximal site of Hfq, whereas the sRNA body, including the seed sequence, is bound by up to six of the lateral sites. In contrast to previously proposed arrangements, the presented model explains how Hfq can protect large parts of the sRNA body while still allowing a rapid recycling of sRNAs. Furthermore, our model suggests molecular mechanisms for the function of Hfq as an RNA chaperone and for the molecular events that are initiated upon mRNA target recognition.

Structural basis for RNA 3'-end recognition by Hfq.

The homohexameric (L)Sm protein Hfq is a central mediator of small RNA-based gene regulation in bacteria. Hfq recognizes small regulatory RNAs (sRNAs) specifically, despite their structural diversity. This specificity could not be explained by previously described RNA-binding modes of Hfq. Here we present a distinct and preferred mode of Hfq-RNA interaction that involves the direct recognition of a uridine-rich RNA 3' end. This feature is common in bacterial RNA transcripts as a consequence of Rho-independent transcription termination and hence likely contributes significantly to the general recognition of sRNAs by Hfq. Isothermal titration calorimetry shows nanomolar affinity between Salmonella typhimurium Hfq and a hexauridine substrate. We determined a crystal structure of the complex that reveals a constricted RNA backbone conformation in the proximal RNA-binding site of Hfq, allowing for a direct protein contact of the 3' hydroxyl group. A free 3' hydroxyl group is crucial for the high-affinity interaction with Hfq also in the context of a full-length sRNA substrate, RybB. The capacity of Hfq to occupy and sequester the RNA 3' end has important implications for the mechanisms by which Hfq is thought to affect sRNA stability, turnover, and regulation.