SummaryNuclear pore complexes (NPCs) are fundamental components of all eukaryotic cells that mediate nucleocytoplasmic exchange. Elucidating their 110 MDa structure imposes a formidable challenge and requires in situ structural biology approaches. Fifteen out of about thirty nucleoporins (Nups) are structured and form the Y- and inner ring complexes. These two major scaffolding modules assemble in multiple copies into an eight-fold rotationally symmetric structure that fuses the inner and outer nuclear membranes to form a central channel of ∼60 nm in diameter 1. The scaffold is decorated with transport channel Nups that often contain phenylalanine (FG)-repeat sequences and mediate the interaction with cargo complexes. Although the architectural arrangement of parts of the Y-complex has been elucidated, it is unclear how exactly it oligomerizes in situ. Here, we combined cryo electron tomography with mass spectrometry, biochemical analysis, perturbation experiments and structural modeling to generate the most comprehensive architectural model of the NPC to date. Our data suggest previously unknown protein interfaces across Y-complexes and to inner ring complex members. We demonstrate that the higher eukaryotic transport channel Nup358 (RanBP2) has a previously unanticipated role in Y-complex oligomerization. Our findings blur the established boundaries between scaffold and transport channel Nups. We conclude that, similarly to coated vesicles, multiple copies of the same structural building block - although compositionally identical - engage in different local sets of interactions and conformations.

The nuclear pore complex (NPC) is a fundamental component of all eukaryotic cells that facilitates nucleocytoplasmic exchange of macromolecules. It is assembled from multiple copies of about 30 nucleoporins. Due to its size and complex composition, determining the structure of the NPC is an enormous challenge, and the overall architecture of the NPC scaffold remains elusive. In this study, we have used an integrated approach based on electron tomography, single-particle electron microscopy, and crosslinking mass spectrometry to determine the structure of a major scaffold motif of the human NPC, the Nup107 subcomplex, in both isolation and integrated into the NPC. We show that 32 copies of the Nup107 subcomplex assemble into two reticulated rings, one each at the cytoplasmic and nuclear face of the NPC. This arrangement may explain how changes of the diameter are realized that would accommodate transport of huge cargoes.

Nuclear pore complexes (NPCs) are 110-megadalton assemblies that mediate nucleocytoplasmic transport. NPCs are built from multiple copies of ~30 different nucleoporins, and understanding how these nucleoporins assemble into the NPC scaffold imposes a formidable challenge. Recently, it has been shown how the Y complex, a prominent NPC module, forms the outer rings of the nuclear pore. However, the organization of the inner ring has remained unknown until now. We used molecular modeling combined with cross-linking mass spectrometry and cryo-electron tomography to obtain a composite structure of the inner ring. This architectural map explains the vast majority of the electron density of the scaffold. We conclude that despite obvious differences in morphology and composition, the higher-order structure of the inner and outer rings is unexpectedly similar.

We describe a method for rapid identification of protein kinase substrates. Cdk1 was engineered to accept an ATP analog that allows it to uniquely label its substrates with a bio-orthogonal phosphate analog tag. A highly specific, covalent capture-andrelease methodology was developed for rapid purification of tagged peptides derived from labeled substrate proteins. Application of this approach to the discovery of Cdk1-cyclin B substrates yielded identification of >70 substrates and phosphorylation sites. Many of these sites are known to be phosphorylated in vivo, but most of the proteins have not been characterized as Cdk1-cyclin B substrates. This approach has the potential to expand our understanding of kinase-substrate connections in signaling networks.P rotein kinases regulate a vast array of biological processes through phosphorylation of protein substrates. A comprehensive map of all phosphorylation sites and kinase-substrate pairs would greatly facilitate the study of signaling networks. This goal faces two fundamental challenges. First, all protein kinases use ATP as a cofactor to phosphorylate their targets, and thus the direct substrates of a single kinase cannot be easily traced in protein mixtures containing multiple kinases. Second, phosphorylation often occurs at low stoichiometry and on low-abundance proteins. This makes substrate and phosphorylation site identification very challenging. Powerful methods have been developed to address these dual problems (1, 2). Kinase-substrate pairs can be tested in multiplexed phosphorylation assays by using immobilized arrays of purified proteins. These high-throughput chip-based assays have the added benefit of presenting low-abundance proteins at easily detectable levels and have provided a first-generation map of protein phosphorylation in Saccharomyces cerevisiae (3). However, these assays do not currently allow identification of phosphorylation sites and have not been adapted to organisms with more complex proteomes. Prediction of high-likelihood kinase substrates can sometimes be achieved by using knowledge of the substrate sequence motif preferences of individual kinases, but only when these preferences are strong (4, 5). Finally, thousands of in vivo phosphorylation sites from metazoan organisms have been identified in proteomic screens (6-9), but for most of these sites the responsible upstream kinases remain unknown.We have demonstrated a chemical and genetic solution to the common use of ATP by all kinases. Our approach relies on engineering a kinase to accept unnatural ATP analogs by modification of the ATP-binding pocket (10, 11). The analogs are very poor substrates for wild-type kinases; thus, an analog-sensitive kinase (or as-kinase) can be used to specifically radiolabel its substrates in cell extracts while preserving important aspects of biological context, such as the integrity of protein complexes. Coupling this approach with the use of libraries of genetically encoded affinity-tagged proteins has facilitated identification of low-abundanc...

The nuclear pore complex (NPC) mediates macromolecular transport between the nucleus and the cytoplasm. Many NPC proteins (nucleoporins, Nups) are modified by phosphorylation. It is believed that phosphorylation regulates the breakdown of the nuclear envelope at mitosis and the disassembly of the NPC into different subcomplexes. In this study, we examined the cell-cycledependent phosphorylation of the Nup107-160 subcomplex, a core building block of the NPC. Using in vivo 32 P labeling in HeLa cells, we found that Nup107, Nup96, and Nup133 are phosphorylated during mitosis. To precisely map the phosphorylation sites within the complex, we used a comprehensive multiple-stage MS approach (MS, MS 2 , and MS 3 ), establishing that Nup160, Nup133, Nup96, and Nup107 are all targets of phosphorylation. We determined that the phosphorylation sites are clustered mainly at the N-terminal regions of these proteins, which are predicted to be natively disordered. In addition, we determined the cell-cycle dependence of the phosphorylation of these sites by using stable isotope labeling and MS 2 analysis. Measurement of the site-specific phosphorylation ratios between mitotic and G1 cells led us to conclude that several phosphorylation events of the subcomplex are mainly mitotic. Based on these results and our finding that the entire Nup107-160 subcomplex is stable throughout the cell cycle, we propose that phosphorylation does not affect interactions within the Nup107-160 subcomplex, but regulates the association of the subcomplex with the NPC and other proteins. mitosis ͉ nucleoporin ͉ mass spectrometry ͉ nuclear pore complex ͉ mammalian