Introduction The nuclear pore complex (NPC) is the primary gateway for transport of macromolecules between the nucleus and cytoplasm, serving as both a critical mediator and regulator of gene expression. NPCs are enormous ~120 MDa macromolecular machines embedded in the nuclear envelope, each containing ~1000 protein subunits, termed nucleoporins. Despite substantial progress in visualizing the overall shape of the NPC by cryoelectron tomography and in determining atomic resolution crystal structures of nucleoporins, the molecular architecture of the assembled NPC remains poorly understood, hindering the design of mechanistic studies that could investigate its many roles in cell biology. Rationale Existing cryoelectron tomographic reconstructions of the NPC remain too low in resolution to allow for de novo structure determination of the NPC or unbiased docking of nucleoporin fragment crystal structures. We sought to bridge this resolution gap by first defining the interaction network of the NPC, focusing on the evolutionarily conserved symmetric core. We developed protocols to reconstitute NPC protomers from purified, recombinant proteins, which enabled the generation of a high-resolution biochemical interaction map of the NPC symmetric core. We next determined high-resolution crystal structures of key nucleoporin interactions, providing spatial restraints for their relative orientation. Lastly, by superposing crystal structures that overlapped in sequence, we generated accurate full-length structures of the large scaffold nucleoporins. Supported by this biochemical data, we used sequential, unbiased searches to place the nucleoporin crystal structures into a previously determined cryoelectron tomographic reconstruction of the intact human NPC, thus generating a composite structure of the entire NPC symmetric core. Results Our analysis revealed that the inner and outer rings of the NPC utilize disparate mechanisms of interaction. While the structured coat nucleoporins of the outer ring form extensive surface contacts, the scaffold proteins of the inner ring are bridged by flexible sequences in linker nucleoporins. Our composite structure revealed a defined spoke architecture with limited cross-spoke interactions. Most nucleoporins are present in 32 copies, with notable exceptions of Nup170 and Nup188. Lastly, we observed the arrangement of the channel nucleoporins, which orient their N-termini into two sixteen-membered rings, ensuring that their N-terminal FG repeats project evenly into the central transport channel. Conclusion Our composite structure of the NPC symmetric core can be used as a platform for the rational design of experiments to probe NPC structure and function. Each nucleoporin occupies multiple distinct biochemical environments, explaining how such a large macromolecular complex can be assembled from a relatively small number of unique genes. Our integrated, bottom-up approach provides a paradigm for the biochemical and structural characterization of similarly large biological mega-assemb...

For transcription initiation, RNA polymerase (Pol) II assembles with general transcription factors on promoter DNA to form the pre-initiation complex (PIC). We report cryo-EM structures of the yeast PIC and PIC-core Mediator (cMed) complex at nominal resolutions of 4.7 Å and 5.8 Å, respectively. The structures reveal TFIIH and suggest how the TFIIH modules ‘core’ and ‘kinase’ function in promoter opening and Pol II phosphorylation, respectively. The TFIIH core subunit Ssl2 (human XPB) is positioned on downstream DNA by the ‘E-bridge’ helix in TFIIE, consistent with TFIIE-stimulated DNA opening. The TFIIH kinase module subunit Tfb3 (human MAT1) anchors the kinase Kin28 (human Cdk7) that is mobile in the PIC but preferentially located between the Mediator hook and shoulder in the PIC-cMed complex. Open spaces between the Mediator head and middle modules may allow access of the kinase to its substrate, the C-terminal domain (CTD) of Pol II.

The nuclear pore complex (NPC) constitutes the sole gateway for bidirectional nucleocytoplasmic transport. We present the reconstitution and interdisciplinary analyses of the ~425-kDa inner ring complex (IRC), which forms the central transport channel and diffusion barrier of the NPC, revealing its interaction network and equimolar stoichiometry. The Nsp1•Nup49•Nup57 channel nucleoporin hetero-trimer (CNT) attaches to the IRC solely through the adaptor nucleoporin Nic96. The CNT•Nic96 structure reveals that Nic96 functions as an assembly sensor that recognizes the three dimensional architecture of the CNT, thereby mediating the incorporation of a defined CNT state into the NPC. We propose that the IRC adopts a relatively rigid scaffold that recruits the CNT to primarily form the diffusion barrier of the NPC, rather than enabling channel dilation.

A key function of the cellular DNA damage response is to facilitate the bypass of replication fork-stalling DNA lesions. Template switch reactions allow such a bypass and involve the formation of DNA joint molecules (JMs) between sister chromatids. These JMs need to be resolved before cell division; however, the regulation of this process is only poorly understood. Here, we identify a regulatory mechanism in yeast that critically controls JM resolution by the Mus81-Mms4 endonuclease. Central to this regulation is a conserved complex comprising the scaffold proteins Dpb11 and Slx4 that is under stringent control. Cell cycle-dependent phosphorylation of Slx4 by Cdk1 promotes the Dpb11-Slx4 interaction, while in mitosis, phosphorylation of Mms4 by Polo-like kinase Cdc5 promotes the additional association of Mus81-Mms4 with the complex, thereby promoting JM resolution. Finally, the DNA damage checkpoint counteracts Mus81-Mms4 binding to the Dpb11-Slx4 complex. Thus, Dpb11-Slx4 integrates several cellular inputs and participates in the temporal program for activation of the JM-resolving nuclease Mus81. Intrinsically and extrinsically induced DNA lesions can compromise the integrity of the genetic information and threaten cell viability. DNA lesions are particularly dangerous during S phase, when faithful DNA replication relies on two intact DNA strands. DNA lesions hamper the progression of replication forks and thereby the complete duplication of chromosomes. Moreover, replication forks that are stalled at DNA lesion sites can collapse and cause chromosome breaks and genome instability (Branzei and Foiani 2010).Eukaryotes possess two fundamentally different mechanisms to bypass DNA lesions that affect one of the parental DNA strands: translesion synthesis (TLS) and template switching. TLS employs specialized polymerases (translesion polymerases) that in many cases are able to replicate the damaged strand but with a reduced fidelity (Prakash et al. 2005). On the other hand, during template switching, the genetic information is copied from the newly synthesized, undamaged sister chromatid. This mechanism is therefore error-free in principle, yet its precise mechanism remains poorly understood. Template switching is a complex process that can be initiated by different recombination-based mechanisms (homologous recombination [HR] and error-free post-replicative repair [PRR]) (Branzei et al. 2008). The choice between the different bypass mechanisms is regulated by ubiquitin and SUMO modifications Ó 2014 Gritenaite et al. This article is distributed exclusively by Cold Spring Harbor Laboratory Press for the first six months after the full-issue publication date (see http://genesdev.cshlp.org/site/misc/terms.xhtml).

Highlights d Promoter meltability defines requirement of TFIIH for initiation d DNA distortions are induced by clamp closure prior to DNA melting d The initially melted region is structurally pre-defined by DNA distortions d Clamp closure and DNA distortion are general features of multi-subunit RNAPs