Monitoring Spatiotemporal Biogenesis of Macromolecular Assemblies by Pulse-Chase Epitope Labeling

Puzzling out nuclear pore complex assembly

Nuclear pore complexes (NPCs) are sophisticated multiprotein assemblies embedded within the nuclear envelope and controlling the exchanges of molecules between the cytoplasm and the nucleus. In this review, we summarize the mechanisms by which these elaborate complexes are built from their subunits, the nucleoporins, based on our ever-growing knowledge of NPC structural organization and on the recent identification of additional features of this process. We present the constraints faced during the production of nucleoporins, their gathering into oligomeric complexes, and the formation of NPCs within nuclear envelopes, and review the cellular strategies at play, from co-translational assembly to the enrolment of a panel of cofactors. Remarkably, the study of NPCs can inform our perception of the biogenesis of multiprotein complexes in general - and vice versa.

Assays for Autophagy III: Observing Dynamic Protein Trafficking

Macroautophagy, by its very nature, is a protein trafficking process. Cargos are transported and processed. Atg proteins come and go. In this chapter, we present three assays to monitor these dynamic events: a non-radioactive pulse-chase labeling assay to monitor the transport of prApe1 and two fluorescent microscopy-based assays to assess the trafficking of Atg8 and Atg9.

A Validated Set of Fluorescent-Protein-Based Markers for Major Organelles in Yeast (Saccharomyces cerevisiae)

Eukaryotic cells share a basic scheme of internal organization featuring membrane-based organelles. The use of fluorescent proteins (FPs) greatly facilitated live-cell imaging of organelle dynamics and protein trafficking. One major limitation of this approach is that the fusion of an FP to a target protein can and often does compromise the function of the target protein and alter its subcellular localization. The optimization process to obtain a desirable fusion construct can be time-consuming or even unsuccessful. In this work, we set out to provide a validated set of FP-based markers for major organelles in the budding yeast (Saccharomyces cerevisiae). Out of over 160 plasmids constructed, we present a final set of 42 plasmids, the recommendations for which are backed up by meticulous evaluations. The tool set includes three colors (green, red, and blue) and covers the endoplasmic reticulum (ER), nucleus, Golgi apparatus, endosomes, vacuoles, mitochondria, peroxisomes, and lipid droplets. The fidelity of the markers was established by systematic cross-comparison and quantification. Functional assays were performed to examine the impact of marker expression on the secretory pathway, endocytic pathway, and metabolic activities of mitochondria and peroxisomes. Concomitantly, our work constitutes a reassessment of organelle identities in this model organism. Our data support the recognition that "late Golgi" and "early endosomes," two seemingly distinct terms, denote the same compartment in yeast. Conversely, all other organelles can be visually separated from each other at the resolution of conventional light microscopy, and quantification results justify their classification as distinct entities.IMPORTANCE Cells contain elaborate internal structures. For eukaryotic cells, like those in our bodies, the internal space is compartmentalized into membrane-bound organelles, each tasked with specialized functions. Oftentimes, one needs to visualize organelles to understand a complex cellular process. Here, we provide a validated set of fluorescent protein-based markers for major organelles in budding yeast. Yeast is a commonly used model when investigating basic mechanisms shared among eukaryotes. Fluorescent proteins are produced by cells themselves, avoiding the need for expensive chemical dyes. Through extensive cross-comparison, we make sure that each of our markers labels and only labels the intended organelle. We also carefully examined if the presence of our markers has any negative impact on the functionality of the cells and found none. Our work also helps answer a related question: are the structures we see really what we think they are?

Viewing pre-60S maturation at a minute's timescale

The formation of ribosomal subunits is a highly dynamic process that is initiated in the nucleus and involves more than 200 trans-acting factors, some of which accompany the pre-ribosomes into the cytoplasm and have to be recycled into the nucleus. The inhibitor diazaborine prevents cytoplasmic release and recycling of shuttling pre-60S maturation factors by inhibiting the AAA-ATPase Drg1. The failure to recycle these proteins results in their depletion in the nucleolus and halts the pathway at an early maturation step. Here, we made use of the fast onset of inhibition by diazaborine to chase the maturation path in real-time from 27SA2 pre-rRNA containing pre-ribosomes localized in the nucleolus up to nearly mature 60S subunits shortly after their export into the cytoplasm. This allows for the first time to put protein assembly and disassembly reactions as well as pre-rRNA processing into a chronological context unraveling temporal and functional linkages during ribosome maturation.

A short linear motif in scaffold Nup145C connects Y-complex with pre-assembled outer ring Nup82 complex

Nucleocytoplasmic transport occurs through nuclear pore complexes (NPCs), which are formed from multiple copies of ~30 different nucleoporins (Nups) and inserted into the double nuclear membrane. Many of these Nups are organized into subcomplexes, of which the Y-shaped Nup84 complex is the major constituent of the nuclear and cytoplasmic rings. The Nup82–Nup159–Nsp1 complex is another module that, however, is only assembled into the cytoplasmic ring. By means of crosslinking mass spectrometry, biochemical reconstitution, and molecular modeling, we identified a short linear motif in the unstructured N-terminal region of Chaetomium thermophilum Nup145C, a subunit of the Y-complex, that is sufficient to recruit the Nup82 complex, but only in its assembled state. This finding points to a more general mechanism that short linear motifs in structural Nups can act as sensors to cooperatively connect pre-assembled NPC modules, thereby facilitating the formation and regulation of the higher-order NPC assembly.

The Nup82–Nup159–Nsp1 complex, which plays a key role in mRNA export, is recruited late during the process of nuclear pore complex (NPC) assembly. Here the authors combine crosslinking mass spectrometry, biochemical reconstitution and molecular modeling to gain insights into the mechanism of Nup82 recruitment to the NPC.

Split-BioID a conditional proteomics approach to monitor the composition of spatiotemporally defined protein complexes

Understanding the function of the thousands of cellular proteins is a central question in molecular cell biology. As proteins are typically part of multiple dynamic and often overlapping macromolecular complexes exerting distinct functions, the identification of protein–protein interactions (PPI) and their assignment to specific complexes is a crucial but challenging task. We present a protein fragments complementation assay integrated with the proximity-dependent biotinylation technique BioID. Activated on the interaction of two proteins, split-BioID is a conditional proteomics approach that allows in a single and simple assay to both experimentally validate binary PPI and to unbiasedly identify additional interacting factors. Applying our method to the miRNA-mediated silencing pathway, we can probe the proteomes of two distinct functional complexes containing the Ago2 protein and uncover the protein GIGYF2 as a regulator of miRNA-mediated translation repression. Hence, we provide a novel tool to study dynamic spatiotemporally defined protein complexes in their native cellular environment.

The BioID approaches takes advantage of the promiscuous biotinylation enzyme (BirA*) to identify proteins that closely interact. Here the authors improve the resolution of BioID using a protein fragment complementation approach that allows the assignment of protein-protein interactions to specific complexes within a common interactome.

Linker 2 of the eukaryotic pre-ribosomal processing factor Mrd1p is an essential interdomain functionally coupled to upstream RNA Binding Domain 2 (RBD2)

Ribosome synthesis is an essential process in all cells. In Sacharomyces cerevisiae, the precursor rRNA, 35S pre-rRNA, is folded and assembled into a 90S pre-ribosomal complex. The 40S ribosomal subunit is processed from the pre-ribosomal complex. This requires concerted action of small nucleolar RNAs, such as U3 snoRNA, and a large number of trans-acting factors. Mrd1p, one of the essential small ribosomal subunit synthesis factors is required for cleavage of the 35S pre-rRNA to generate 18S rRNA of the small ribosomal subunit. Mrd1p is evolutionary conserved in all eukaryotes and in yeast it contains five RNA Binding Domains (RBDs) separated by linker regions. One of these linkers, Linker 2 between RBD2 and RBD3, is conserved in length, predicted to be structured and contains conserved clusters of amino acid residues. In this report, we have analysed Linker 2 mutations and demonstrate that it is essential for Mrd1p function during pre-ribosomal processing. Extensive changes of amino acid residues as well as specific changes of conserved clusters of amino acid residues were found to be incompatible with synthesis of pre-40S ribosomes and cell growth. In addition, gross changes in primary sequence of Linker 2 resulted in Mrd1p instability, leading to degradation of the N-terminal part of the protein. Our data indicates that Linker 2 is functionally coupled to RBD2 and argues for that these domains constitute a functional module in Mrd1p. We conclude that Linker 2 has an essential role for Mrd1p beyond just providing a defined length between RBD2 and RBD3.

Hold on to your friends: Dedicated chaperones of ribosomal proteins: Dedicated chaperones mediate the safe transfer of ribosomal proteins to their site of pre-ribosome incorporation

Eukaryotic ribosomes are assembled from their components, the ribosomal RNAs and ribosomal proteins, in a tremendously complex, multi-step process, which primarily takes place in the nuclear compartment. Therefore, most ribosomal proteins have to travel from the cytoplasm to their incorporation site on pre-ribosomes within the nucleus. However, due to their particular characteristics, such as a highly basic amino acid composition and the presence of unstructured extensions, ribosomal proteins are especially prone to aggregation and degradation in their unassembled state, hence specific mechanisms must operate to ensure their safe delivery. Recent studies have uncovered a group of proteins, termed dedicated chaperones, specialized in accompanying and guarding individual ribosomal proteins. In this essay, we review how these dedicated chaperones utilize different folds to interact with their ribosomal protein clients and how they ensure their soluble expression and interconnect their intracellular transport with their efficient assembly into pre-ribosomes.

The Anchored Flexibility Model in LC8 Motif Recognition: Insights from the Chica Complex

LC8 is a dimeric hub protein involved in a large number of interactions central to cell function. It binds short linear motifs--usually containing a Thr-Gln-Thr (TQT) triplet--in intrinsically disordered regions of its binding partners, some of which have several LC8 recognition motifs in tandem. Hallmarks of the 7-10 amino acid motif are a high variability of LC8 binding affinity and extensive sequence permutation outside the TQT triplet. To elucidate the molecular basis of motif recognition, we use a 69-residue segment of the human Chica spindle adaptor protein that contains four putative TQT recognition motifs in tandem. NMR-derived secondary chemical shifts and relaxation properties show that the Chica LC8 binding domain is essentially disordered with a dynamically restricted segment in one linker between motifs. Calorimetry of LC8 binding to synthetic motif-mimicking peptides shows that the first motif dominates LC8 recruitment. Crystal structures of the complexes of LC8 bound to each of two motif peptides show highly ordered and invariant TQT-LC8 interactions and more flexible and conformationally variable non-TQT-LC8 interactions. These data highlight rigidity in both LC8 residues that bind TQT and in the TQT portion of the motif as an important new characteristic of LC8 recognition. On the basis of these data and others in the literature, we propose that LC8 recognition is based on rigidly fixed interactions between LC8 and TQT residues that act as an anchor, coupled with inherently flexible interactions between LC8 and non-TQT residues. The "anchored flexibility" model explains the requirement for the TQT triplet and the ability of LC8 to accommodate a large variety of motif sequences and affinities.

The Ccl1-Kin28 kinase complex regulates autophagy under nitrogen starvation

Starvation triggers global alterations in the synthesis and turnover of proteins. Under such conditions, the recycling of essential nutrients by using autophagy is indispensable for survival. By screening known kinases in the yeast genome, we newly identified a regulator of autophagy, the Ccl1-Kin28 kinase complex (the equivalent of the mammalian cyclin-H-Cdk7 complex), which is known to play key roles in RNA-polymerase-II-mediated transcription. We show that inactivation of Ccl1 caused complete block of autophagy. Interestingly, Ccl1 itself was subject to proteasomal degradation, limiting the level of autophagy during prolonged starvation. We present further evidence that the Ccl1-Kin28 complex regulates the expression of Atg29 and Atg31, which is crucial in the assembly of the Atg1 kinase complex. The identification of this previously unknown regulatory pathway sheds new light on the complex signaling network that governs autophagy activity.

The Dedicated Chaperone Acl4 Escorts Ribosomal Protein Rpl4 to Its Nuclear Pre-60S Assembly Site

Ribosomes are the highly complex macromolecular assemblies dedicated to the synthesis of all cellular proteins from mRNA templates. The main principles underlying the making of ribosomes are conserved across eukaryotic organisms and this process has been studied in most detail in the yeast Saccharomyces cerevisiae. Yeast ribosomes are composed of four ribosomal RNAs (rRNAs) and 79 ribosomal proteins (r-proteins). Most r-proteins need to be transported from the cytoplasm to the nucleus where they get incorporated into the evolving pre-ribosomal particles. Due to the high abundance and difficult physicochemical properties of r-proteins, their correct folding and fail-safe targeting to the assembly site depends largely on general, as well as highly specialized, chaperone and transport systems. Many r-proteins contain universally conserved or eukaryote-specific internal loops and/or terminal extensions, which were shown to mediate their nuclear targeting and association with dedicated chaperones in a growing number of cases. The 60S r-protein Rpl4 is particularly interesting since it harbours a conserved long internal loop and a prominent C-terminal eukaryote-specific extension. Here we show that both the long internal loop and the C-terminal eukaryote-specific extension are strictly required for the functionality of Rpl4. While Rpl4 contains at least five distinct nuclear localization signals (NLS), the C-terminal part of the long internal loop associates with a specific binding partner, termed Acl4. Absence of Acl4 confers a severe slow-growth phenotype and a deficiency in the production of 60S subunits. Genetic and biochemical evidence indicates that Acl4 can be considered as a dedicated chaperone of Rpl4. Notably, Acl4 localizes to both the cytoplasm and nucleus and it has the capacity to capture nascent Rpl4 in a co-translational manner. Taken together, our findings indicate that the dedicated chaperone Acl4 accompanies Rpl4 from the cytoplasm to its pre-60S assembly site in the nucleus.

Emerging properties of nuclear RNP biogenesis and export

RNA biology has recently seen an explosion of data due to advances in RNA sequencing, proteomic, and RNA imaging technologies. In this review, we highlight progress that has been made using these approaches in the area of nuclear RNP biogenesis and export. Excitingly, the ability to collect quantitative data at the 'omics' scale combined with measurements of transcription, decay, and transport kinetics is providing the information needed to address RNP biogenesis at a systems level. We believe this to be a necessary and critical next step that will lead to a better understanding of how RNP quality, diversity, and fate emerge from a defined set of nuclear RNP assembly and maturation steps.

Coordinated Ribosomal L4 Protein Assembly into the Pre-Ribosome Is Regulated by Its Eukaryote-Specific Extension

Eukaryotic ribosome biogenesis requires nuclear import and hierarchical incorporation of ∼80 ribosomal proteins (RPs) into the ribosomal RNA core. In contrast to prokaryotes, many eukaryotic RPs possess long extensions that interdigitate in the mature ribosome. RpL4 is a prime example, with an ∼80-residue-long surface extension of unknown function. Here, we identify assembly chaperone Acl4 that initially binds the universally conserved internal loop of newly synthesized RpL4 via its superhelical TPR domain, thereby restricting RpL4 loop insertion at its cognate nascent rRNA site. RpL4 release from Acl4 is orchestrated with pre-ribosome assembly, during which the eukaryote-specific RpL4 extension makes several distinct interactions with the 60S surface, including a co-evolved site on neighboring RpL18. Consequently, mutational inactivation of this contact site, on either RpL4 or RpL18, impairs RpL4-Acl4 disassembly and RpL4 pre-ribosome incorporation. We propose that hierarchical ribosome assembly can be achieved by eukaryotic RP extensions and dedicated assembly chaperones.

Structural basis for assembly and function of the Nup82 complex in the nuclear pore scaffold

The yeast Nup82 complex forms an unusual asymmetric structure with a dimeric array of subunits that mediate its anchorage to the NPC scaffold and its concomitant interaction with the soluble nucleocytoplasmic transport machinery.

Nuclear pore complexes (NPCs) are huge assemblies formed from ∼30 different nucleoporins, typically organized in subcomplexes. One module, the conserved Nup82 complex at the cytoplasmic face of NPCs, is crucial to terminate mRNA export. To gain insight into the structure, assembly, and function of the cytoplasmic pore filaments, we reconstituted in yeast the Nup82–Nup159–Nsp1–Dyn2 complex, which was suitable for biochemical, biophysical, and electron microscopy analyses. Our integrative approach revealed that the yeast Nup82 complex forms an unusual asymmetric structure with a dimeric array of subunits. Based on all these data, we developed a three-dimensional structural model of the Nup82 complex that depicts how this module might be anchored to the NPC scaffold and concomitantly can interact with the soluble nucleocytoplasmic transport machinery.

Paradigms of ribosome synthesis: Lessons learned from ribosomal proteins

The proteome in all cells is manufactured via the intricate process of translation by multimolecular factories called ribosomes. Nevertheless, these ribonucleoprotein particles, the largest of their kind, also have an elaborate assembly line of their own. Groundbreaking discoveries that bacterial ribosomal subunits can be self-assembled in vitro jumpstarted studies on how ribosomes are constructed. Until recently, ribosome assembly has been investigated almost entirely in vitro with bacterial small subunits under equilibrium conditions. In light of high-resolution ribosome structures and a more sophisticated toolkit, the past decade has been defined by a burst of kinetic studies in vitro and, importantly, also a shift to examining ribosome maturation in living cells, especially in eukaryotes. In this review, we summarize the principles governing ribosome assembly that emerged from studies focusing on ribosomal proteins and their interactions with rRNA. Understanding these paradigms has taken center stage, given the linkage between anomalous ribosome biogenesis and proliferative disorders.

A network of assembly factors is involved in remodeling rRNA elements during preribosome maturation

Eukaryotic ribosome biogenesis involves ∼200 assembly factors, but how these contribute to ribosome maturation is poorly understood. Here, we identify a network of factors on the nascent 60S subunit that actively remodels preribosome structure. At its hub is Rsa4, a direct substrate of the force-generating ATPase Rea1. We show that Rsa4 is connected to the central protuberance by binding to Rpl5 and to ribosomal RNA (rRNA) helix 89 of the nascent peptidyl transferase center (PTC) through Nsa2. Importantly, Nsa2 binds to helix 89 before relocation of helix 89 to the PTC. Structure-based mutations of these factors reveal the functional importance of their interactions for ribosome assembly. Thus, Rsa4 is held tightly in the preribosome and can serve as a "distribution box," transmitting remodeling energy from Rea1 into the developing ribosome. We suggest that a relay-like factor network coupled to a mechano-enzyme is strategically positioned to relocate rRNA elements during ribosome maturation.

Pulse-chase analysis for studying protein synthesis and maturation

Pulse-chase analysis is a well-established and highly adaptable tool for studying the life cycle of endogenous proteins, including their synthesis, folding, subunit assembly, intracellular transport, post-translational processing, and degradation. This unit describes the performance and analysis of a radiolabel pulse-chase experiment for following the folding and cell surface trafficking of a trimeric murine MHC class I glycoprotein. In particular, the unit focuses on the precise timing of pulse-chase experiments to evaluate early/short-time events in protein maturation in both suspended and strictly adherent cell lines. The advantages and limitations of radiolabel pulse-chase experiments are discussed, and a comprehensive section for troubleshooting is provided. Further, ways to quantitatively represent pulse-chase results are described, and feasible interpretations on protein maturation are suggested. The protocols can be adapted to investigate a variety of proteins that may mature in very different ways.

A pulse-chase epitope labeling to study cellular dynamics of newly synthesized proteins: a novel strategy to characterize NPC biogenesis and ribosome maturation/export

The vast number of cellular proteins performs their roles within macromolecular assemblies and functional cell networks. Hence, an understanding of how multiprotein complexes are formed and modified during biogenesis is a key problem in cell biology. Here, we describe a detailed protocol for a nonradioactive pulse-chase in vivo-labeling approach. The method is based on the incorporation of an unnatural amino acid (O-methyl-tyrosine) by the nonsense suppression of an amber stop codon that quickly fuses an affinity tag of choice to a protein of interest. This affinity tag could be used to directly isolate the newly synthesized proteins and hence allows for the characterization of early complex biogenesis intermediates. Combined with a tetracycline controllable riboswitch in the 5'-UTR of the respective mRNA, this approach became a versatile tool to study dynamic protein assembly within cellular networks (Stelter et al., 2012). In the context of this volume, this method notably provides a suitable approach to study NPC, ribosome and mRNP biogenesis, or nuclear protein translocation. This chapter includes detailed protocols to track newly synthesized, epitope pulsed-chased proteins by western blot, their assembly within complexes using immunoprecipitation, and their subcellular localization using indirect immunofluorescence or subcellular fractionation. While these protocols use budding yeast as model system, this method can be adapted to other model systems.

Multiple recognition motifs in nucleoporin Nup159 provide a stable and rigid Nup159-Dyn2 assembly

Dyn2 is the yeast ortholog of the molecular hub LC8, which binds disordered proteins and promotes their self-association and higher order assembly. Dyn2 is proposed to dimerize and stabilize the Nup82-Nsp1-Nup159 complex of the nuclear pore assembly through its interaction with nucleoporin Nup159. Nup159 has six LC8 recognition motifs separated by short linkers. NMR experiments reported here show that the Dyn2 binding domain of Nup159 is intrinsically disordered and that binding of one equivalent of Dyn2 dimer aligns two Nup159 chains along the full Dyn2 binding domain to form a bivalent scaffold that promotes binding of other Dyn2 dimers. Isothermal titration calorimetry of Dyn2 binding to Nup constructs of increasing lengths determine that the third LC8 recognition motifs does not bind Dyn2. A new approach to identifying active LC8 recognition motifs based on NMR-detected β-sheet propensities is presented. Isothermal titration calorimetry experiments also show that, due to unfavorable entropy changes, a Nup-Dyn2 complex with three Dyn2 dimers is more stable than the wild-type complex with five Dyn2 dimers. The calorimetric results argue that, from a thermodynamics perspective, only three Dyn2 dimers are needed for optimal stability and suggest that the evolutionary adaptation of multiple tandem LC8 recognition motifs imparts to the complex other properties such as rigidity and a kink in the rod-like structure. These findings extend the repertoire of functions of intrinsically disordered protein to fine-tuning and versatile assembly of higher order macromolecular complexes.