Architecture of human Rag GTPase heterodimers and their complex with mTORC1

© 2019 American Association for the Advancement of Science. All rights reserved. The Rag guanosine triphosphatases (GTPases) recruit the master kinase mTORC1 to lysosomes to regulate cell growth and proliferation in response to amino acid availability. The nucleotide state of Rag heterodimers is critical for their association with mTORC1. Our cryo–electron microscopy structure of RagA/RagC in complex with mTORC1 shows the details of RagA/RagC binding to the RAPTOR subunit of mTORC1 and explains why only the RagAGTP/RagCGDPnucleotide state binds mTORC1. Previous kinetic studies suggested that GTP binding to one Rag locks the heterodimer to prevent GTP binding to the other. Our crystal structures and dynamics of RagA/RagC show the mechanism for this locking and explain how oncogenic hotspot mutations disrupt this process. In contrast to allosteric activation by RHEB, Rag heterodimer binding does not change mTORC1 conformation and activates mTORC1 by targeting it to lysosomes

IP6 is an HIV pocket factor that prevents capsid collapse and promotes DNA synthesis.

The HIV capsid is semipermeable and covered in electropositive pores that are essential for viral DNA synthesis and infection. Here, we show that these pores bind the abundant cellular polyanion IP6, transforming viral stability from minutes to hours and allowing newly synthesised DNA to accumulate inside the capsid. An arginine ring within the pore coordinates IP6, which strengthens capsid hexamers by almost 10°C. Single molecule measurements demonstrate that this renders native HIV capsids highly stable and protected from spontaneous collapse. Moreover, encapsidated reverse transcription assays reveal that, once stabilised by IP6, the accumulation of new viral DNA inside the capsid increases >100 fold. Remarkably, isotopic labelling of inositol in virus-producing cells reveals that HIV selectively packages over 300 IP6 molecules per infectious virion. We propose that HIV recruits IP6 to regulate capsid stability and uncoating, analogous to picornavirus pocket factors. HIV-1/IP6/capsid/co-factor/reverse transcription

Molecular determinants underlying functional innovations of TBP and their impact on transcription initiation.

Funder: RCUK | Medical Research Council (MRC); doi: https://doi.org/10.13039/501100000265TATA-box binding protein (TBP) is required for every single transcription event in archaea and eukaryotes. It binds DNA and harbors two repeats with an internal structural symmetry that show sequence asymmetry. At various times in evolution, TBP has acquired multiple interaction partners and different organisms have evolved TBP paralogs with additional protein regions. Together, these observations raise questions of what molecular determinants (i.e. key residues) led to the ability of TBP to acquire new interactions, resulting in an increasingly complex transcriptional system in eukaryotes. We present a comprehensive study of the evolutionary history of TBP and its interaction partners across all domains of life, including viruses. Our analysis reveals the molecular determinants and suggests a unified and multi-stage evolutionary model for the functional innovations of TBP. These findings highlight how concerted chemical changes on a conserved structural scaffold allow for the emergence of complexity in a fundamental biological process

Redox Signaling by the RNA Polymerase III TFIIB-Related Factor Brf2.

TFIIB-related factor 2 (Brf2) is a member of the family of TFIIB-like core transcription factors. Brf2 recruits RNA polymerase (Pol) III to type III gene-external promoters, including the U6 spliceosomal RNA and selenocysteine tRNA genes. Found only in vertebrates, Brf2 has been linked to tumorigenesis but the underlying mechanisms remain elusive. We have solved crystal structures of a human Brf2-TBP complex bound to natural promoters, obtaining a detailed view of the molecular interactions occurring at Brf2-dependent Pol III promoters and highlighting the general structural and functional conservation of human Pol II and Pol III pre-initiation complexes. Surprisingly, our structural and functional studies unravel a Brf2 redox-sensing module capable of specifically regulating Pol III transcriptional output in living cells. Furthermore, we establish Brf2 as a central redox-sensing transcription factor involved in the oxidative stress pathway and provide a mechanistic model for Brf2 genetic activation in lung and breast cancer

Bipartite binding and partial inhibition links DEPTOR and mTOR in a mutually antagonistic embrace.

The mTORC1 kinase complex regulates cell growth, proliferation, and survival. Because mis-regulation of DEPTOR, an endogenous mTORC1 inhibitor, is associated with some cancers, we reconstituted mTORC1 with DEPTOR to understand its function. We find that DEPTOR is a unique partial mTORC1 inhibitor that may have evolved to preserve feedback inhibition of PI3K. Counterintuitively, mTORC1 activated by RHEB or oncogenic mutation is much more potently inhibited by DEPTOR. Although DEPTOR partially inhibits mTORC1, mTORC1 prevents this inhibition by phosphorylating DEPTOR, a mutual antagonism that requires no exogenous factors. Structural analyses of the mTORC1/DEPTOR complex showed DEPTOR's PDZ domain interacting with the mTOR FAT region, and the unstructured linker preceding the PDZ binding to the mTOR FRB domain. The linker and PDZ form the minimal inhibitory unit, but the N-terminal tandem DEP domains also significantly contribute to inhibition

Structural Basis for PTPA Interaction with the Invariant C-Terminal Tail of PP2A

Protein phosphatase 2A (PP2A) is a highly abundant heterotrimeric Ser/Thr phosphatase involved in the regulation of a variety of signaling pathways. The PP2A phosphatase activator (PTPA) is an ATP-dependent activation chaperone, which plays a key role in the biogenesis of active PP2A. The C-terminal tail of the catalytic subunit of PP2A is highly conserved and can undergo a number of posttranslational modifications that serve to regulate the function of PP2A. Here we have studied structurally the interaction of PTPA with the conserved C-terminal tail of the catalytic subunit carrying different posttranslational modifications. We have identified an additional interaction site for the invariant C-terminal tail of the catalytic subunit on PTPA, which can be modulated via posttranslational modifications. We show that phosphorylation of Tyr307PP2A-C or carboxymethylation of Leu309PP2A-C abrogates or diminishes binding of the C-terminal tail, whereas phosphorylation of Thr304PP2A-C is of no consequence. We suggest that the invariant C-terminal residues of the catalytic subunit can act as affinity enhancer for different PP2A interaction partners, including PTPA, and a different ‘code’ of posttranslational modifications can favour interactions to one subunit over others

Overall structure of PR70.

<p><b>A)</b> Overview of the structure. CBLD1 and 2 are light blue and dark red respectively, the linker between them is violet and calcium ions are yellow. <b>B)</b> Composition of the two calcineurin B-like domains. CBLD1 (left) and CBLD2 (right) are both shown in same orientation as in A, top panel. EF hands are blue (EF1), light blue (EF2), green (EF3), yellow (EF4), pale orange (EF5), dark red (EF6), violet (EF7) and pink (EF8), termini and connecting loops are grey, the C-terminal helix is black, and calcium is yellow. <b>C)</b> Sequence of EF loops 5 and 7. EF loop consensus sequence is shown. Residues conforming to the consensus are highlighted in grey. <b>D)</b> Structure of EF loops 5 and 7. Colored as in B. Water molecules interacting with calcium are red. Side chains are omitted for residues not directly involved in calcium binding (except for Asp-338). <b>E)</b> Structural changes upon complex formation. DynDom suggests the presence of two domains that move relative to each other upon formation of the PP2A holoenzyme. Free and complexed PR70 were superimposed using DynDom domain 1. The stippled line marks the boundary between CBLD1 and 2, and the full line marks the approximate boundary between the DynDom domains. For free PR70, DynDom domain 1 and 2 are pale pink and pink respectively and calcium is yellow. For complexed PR70, DynDom domains 1 and 2 are pale green and turquoise respectively and calcium is orange. <b>F)</b> Structural comparison of free PR70 and free PR72. Orientation as in E. For PR72, DynDom domains 1 and 2 are light blue and blue respectively and calcium is green.</p