Co-translational binding of importins to nascent proteins

Protein folding and quality control during nuclear transport

The spatial separation of protein synthesis from the compartmental destiny of proteins led to the evolution of transport systems that are efficient and yet highly specific. Co-translational transport has emerged as a strategy to avoid cytosolic aggregation of folding intermediates and the need for energy-consuming unfolding strategies to enable transport through narrow conduits connecting compartments. While translation and compartmental translocation are at times tightly coordinated, we know very little about the temporal coordination of translation, protein folding, and nuclear import. Here, we consider the implications of co-translational engagement of nuclear import machinery. We propose that the dynamic interplay of karyopherins and intrinsically disordered nucleoporins create a favorable protein folding environment for cargo en route to the nuclear compartment while maintaining a barrier function of the nuclear pore complex. Our model is discussed in the context of neurological disorders that are tied to defects in nuclear transport and protein quality control.

The SMN-ribosome interplay: a new opportunity for Spinal Muscular Atrophy therapies

The underlying cause of Spinal Muscular Atrophy (SMA) is in the reduction of survival motor neuron (SMN) protein levels due to mutations in the SMN1 gene. The specific effects of SMN protein loss and the resulting pathological alterations are not fully understood. Given the crucial roles of the SMN protein in snRNP biogenesis and its interactions with ribosomes and translation-related proteins and mRNAs, a decrease in SMN levels below a specific threshold in SMA is expected to affect translational control of gene expression. This review covers both direct and indirect SMN interactions across various translation-related cellular compartments and processes, spanning from ribosome biogenesis to local translation and beyond. Additionally, it aims to outline deficiencies and alterations in translation observed in SMA models and patients, while also discussing the implications of the relationship between SMN protein and the translation machinery within the context of current and future therapies.

Nuclear-import receptors as gatekeepers of pathological phase transitions in ALS/FTD

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative disorders on a disease spectrum that are characterized by the cytoplasmic mislocalization and aberrant phase transitions of prion-like RNA-binding proteins (RBPs). The common accumulation of TAR DNA-binding protein-43 (TDP-43), fused in sarcoma (FUS), and other nuclear RBPs in detergent-insoluble aggregates in the cytoplasm of degenerating neurons in ALS/FTD is connected to nuclear pore dysfunction and other defects in the nucleocytoplasmic transport machinery. Recent advances suggest that beyond their canonical role in the nuclear import of protein cargoes, nuclear-import receptors (NIRs) can prevent and reverse aberrant phase transitions of TDP-43, FUS, and related prion-like RBPs and restore their nuclear localization and function. Here, we showcase the NIR family and how they recognize cargo, drive nuclear import, and chaperone prion-like RBPs linked to ALS/FTD. We also discuss the promise of enhancing NIR levels and developing potentiated NIR variants as therapeutic strategies for ALS/FTD and related neurodegenerative proteinopathies.

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.

Changing the guard-nuclear pore complex quality control

The integrity of the nuclear envelope depends on the function of nuclear pore complexes (NPCs), transport channels that control macromolecular traffic between the nucleus and cytosol. The central importance of NPCs suggests the existence of quality control (QC) mechanisms that oversee their assembly and function. In this perspective, we emphasize the challenges associated with NPC assembly and the need for QC mechanisms that operate at various stages of an NPC's life. This includes cytosolic preassembly QC that helps enforce key nucleoporin-nucleoporin interactions and their ultimate stoichiometry in the NPC in addition to mechanisms that monitor aberrant fusion of the inner and outer nuclear membranes. Furthermore, we discuss whether and how these QC mechanisms may operate to sense faulty mature NPCs to facilitate their repair or removal. The so far uncovered mechanisms for NPC QC provide fertile ground for future research that not only benefits a better understanding of the vital role that NPCs play in cellular physiology but also how loss of NPC function and/or these QC mechanisms might be an input to aging and disease.

Coordinating nucleoporin condensation and nuclear pore complex assembly

The nuclear pore complex (NPC) is among the most elaborate protein complexes in eukaryotes. While ribosomes and proteasomes are known to require dedicated assembly machinery, our understanding of NPC assembly is at a relatively early stage. Defects in NPC assembly or homeostasis are tied to movement disorders, including dystonia and amyotrophic lateral sclerosis (ALS), as well as aging, requiring a better understanding of these processes to enable therapeutic intervention. Here, we discuss recent progress in the understanding of NPC assembly and highlight how related defects in human disorders can shed light on NPC biogenesis. We propose that the condensation of phenylalanine-glycine repeat nucleoporins needs to be carefully controlled during NPC assembly to prevent aberrant condensation, aggregation, or amyloid formation.

ATAC and SAGA co-activator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes, it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here, we demonstrate that the core modules of ATAC (ADA-two-A-containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription co-activator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, a SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histone proteins. In contrast, ATAC complex subunits cannot be detected in the cytoplasm of mammalian cells. However, an endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related co-activators, ATAC and SAGA, assemble using co-translational pathways, but their subcellular localization, cytoplasmic abundance, and functions are distinct.

Control of mRNA fate by its encoded nascent polypeptide

Cells tightly regulate mRNA processing, localization, and stability to ensure accurate gene expression in diverse cellular states and conditions. Most of these regulatory steps have traditionally been thought to occur before translation by the action of RNA-binding proteins. Several recent discoveries highlight multiple co-translational mechanisms that modulate mRNA translation, localization, processing, and stability. These mechanisms operate by recognition of the nascent protein, which is necessarily coupled to its encoding mRNA during translation. Hence, the distinctive sequence or structure of a particular nascent chain can recruit recognition factors with privileged access to the corresponding mRNA in an otherwise crowded cellular environment. Here, we draw on both well-established and recent examples to provide a conceptual framework for how cells exploit nascent protein recognition to direct mRNA fate. These mechanisms allow cells to dynamically and specifically regulate their transcriptomes in response to changes in cellular states to maintain protein homeostasis.

ATAC and SAGA coactivator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here we demonstrate that the core modules of ATAC (ADA-Two-A-Containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription coactivator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histones proteins. In contrast, fully assembled ATAC complex cannot be detected in the cytoplasm of mammalian cells. However, endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related coactivators, ATAC and SAGA, assemble by using co-translational pathways, but their subcellular localization, cytoplasmic abundance and functions are distinct.

Hierarchical TAF1-dependent co-translational assembly of the basal transcription factor TFIID

Large heteromeric multiprotein complexes play pivotal roles at every step of gene expression in eukaryotic cells. Among them, the 20-subunit basal transcription factor TFIID nucleates the RNA polymerase II preinitiation complex at gene promoters. Here, by combining systematic RNA-immunoprecipitation (RIP) experiments, single-molecule imaging, proteomics and structure-function analyses, we show that human TFIID biogenesis occurs co-translationally. We discovered that all protein heterodimerization steps happen during protein synthesis. We identify TAF1-the largest protein in the complex-as a critical factor for TFIID assembly. TAF1 acts as a flexible scaffold that drives the co-translational recruitment of TFIID submodules preassembled in the cytoplasm. Altogether, our data suggest a multistep hierarchical model for TFIID biogenesis that culminates with the co-translational assembly of the complex onto the nascent TAF1 polypeptide. We envision that this assembly strategy could be shared with other large heteromeric protein complexes.