The nuclear localization sequence mediates hnRNPA1 amyloid fibril formation revealed by cryoEM structure

Abstract

Human heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) serves as a key regulating protein in RNA metabolism. Malfunction of hnRNPA1 in nucleo-cytoplasmic transport or dynamic phase separation leads to abnormal amyloid aggregation and neurodegeneration. The low complexity (LC) domain of hnRNPA1 drives both dynamic phase separation and amyloid aggregation. Here, we use cryo-electron microscopy to determine the amyloid fibril structure formed by hnRNPA1 LC domain. Remarkably, the structure reveals that the nuclear localization sequence of hnRNPA1 (termed PY-NLS), which is initially known to mediate the nucleo-cytoplamic transport of hnRNPA1 through binding with karyopherin-β2 (Kapβ2), represents the major component of the fibril core. The residues that contribute to the binding of PY-NLS with Kapβ2 also exert key molecular interactions to stabilize the fibril structure. Notably, hnRNPA1 mutations found in familial amyotrophic lateral sclerosis (ALS) and multisystem proteinopathoy (MSP) are all involved in the fibril core and contribute to fibril stability. Our work illuminates structural understandings of the pathological amyloid aggregation of hnRNPA1 and the amyloid disaggregase activity of Kapβ2, and highlights the multiple roles of PY-NLS in hnRNPA1 homeostasis.

Transportin-1: A Nuclear Import Receptor with Moonlighting Functions

“…Thus, in this case, the NLS might serve as a signal that ensures that nuclear cargos are chaperoned and disaggregated when they are trapped in the cytoplasm ( Figure 4 ) ( Guo et al, 2018 ; Hofweber et al, 2018 ; Qamar et al, 2018 ; Yoshizawa et al, 2018 ). Similar mechanisms have been proposed for the Trn1-mediated disaggregation of the amyloid fibrils formed by hnRNP A1 LC domain, thus conferring a protective activity against hnRNP A1-driven ALS and MSP ( Sun et al, 2020 ).…”

“…Recent studies show, that Trn1 not only acts as a transport receptor but presumably serves as a powerful cytoplasmic chaperone for mislocalized RBPs. Thus, Trn1 regulates SGs formation and their further fibrillization ( Figure 4 ) ( Guo et al, 2018 ; Hock et al, 2018 ; Hofweber et al, 2018 ; Qamar et al, 2018 ; Yoshizawa et al, 2018 ; Sun et al, 2020 ).…”

Altered Phase Separation and Cellular Impact in C9orf72-Linked ALS/FTD

“…Further the hydrogel forming phenylalanine-glycine domain of nuclear pore proteins is also thought to form LARKs (Hughes et al, 2018). Other motifs that form reversible amyloid fibrils have also been identified in the LCDs of FUS, hnRNPA1 and hnRNPA2 (Luo et al, 2018;Gui et al, 2019;Lu et al, 2020;Sun et al, 2020b). Hence amyloid formation may have a practical function in membraneless organelle formation, for more stable, less transient structures.…”

hnRNP A/B Proteins: An Encyclopedic Assessment of Their Roles in Homeostasis and Disease

“…Insoluble protein aggregates occur when granules are no longer able to disassemble [178,191]. Biological mechanisms for protein recycling are less effective due to aggregate size and insolubility, and so aggregates accumulate in pathologic fibrils that can lead to cell death and in neurons, neurodegeneration [192][193][194]. For diseases like ALS, A1 and A2/B1-containing pathogenic aggregates can be seeded by accumulation of other PrLD-containing RNA binding proteins (RBPs) including TDP-43, FUS, and TIA1 [178,182,183,195].…”

“…No structural information for A0 nor A3 is available. Because of their relative structural stability, the globular RRM domains of both proteins have been characterized in detail, while researchers have only recently begun to examine the C-terminal LCD (PrLD) region in full [193,297], since the structure of this region is highly variable. Since this region is also less well-conserved relative to the RRMs, application of A1-derived structures to interpretation of A2/B1 and vice versa, or to the other hnRNP A/B family proteins, requires some caution.…”