Abstract
hnRNPA2 is a human ribonucleoprotein (RNP) involved in RNA metabolism. It forms fibrils both under cellular stress and in mutated form in neurodegenerative conditions. Previous work established that the C-terminal low-complexity domain (LCD) of hnRNPA2 fibrillizes under stress, and missense mutations in this domain are found in the disease multisystem proteinopathy (MSP). However, little is known at the atomic level about the hnRNPA2 LCD structure that is involved in those processes and how disease mutations cause structural change. Here we present the cryo-electron microscopy (cryoEM) structure of the hnRNPA2 LCD fibril core and demonstrate its capability to form a reversible hydrogel in vitro containing amyloid-like fibrils. Whereas these fibrils, like pathogenic amyloid, are formed from protein chains stacked into β-sheets by backbone hydrogen bonds, they display distinct structural differences: the chains are kinked, enabling non-covalent cross-linking of fibrils and disfavoring formation of pathogenic steric zippers. Both reversibility and energetic calculations suggest these fibrils are less stable than pathogenic amyloid. Moreover, the crystal structure of the disease-mutation-containing segment (D290V) of hnRNPA2 suggests that the replacement fundamentally alters the fibril structure to a more stable energetic state. These findings illuminate how molecular interactions promote protein fibril networks and how mutation can transform fibril structure from functional to a pathogenic form.
Highlights
HnRNPA2 is a human ribonucleoprotein (RNP) involved in RNA metabolism
Negativestain transmission electron microscopy (TEM) of the diluted hydrogel showed a network of uniform amyloid-like fibrils with an average width of 20 nm (Fig. 1b)
The X-ray diffraction pattern is consistent with cross-β architecture: two reflections at 4.7 and 10 Å corresponding to the inter-strand and inter-sheet spacing, respectively[17], of amyloid fibrils (Fig. 1c)
Summary
HnRNPA2 is a human ribonucleoprotein (RNP) involved in RNA metabolism. It forms fibrils both under cellular stress and in mutated form in neurodegenerative conditions. We present the cryo-electron microscopy (cryoEM) structure of the hnRNPA2 LCD fibril core and demonstrate its capability to form a reversible hydrogel in vitro containing amyloid-like fibrils Whereas these fibrils, like pathogenic amyloid, are formed from protein chains stacked into β-sheets by backbone hydrogen bonds, they display distinct structural differences: the chains are kinked, enabling non-covalent cross-linking of fibrils and disfavoring formation of pathogenic steric zippers. Other mutations found in the RNP LCDs were previously shown to impede MLO and hydrogel formation[1,11,12] Informing both functional MLO-associated fibrils and pathogenic disease-associated fibrils, hnRNPA2 is similar to other RNA-binding proteins, including hnRNPA113, FUS7,9,12,14, TIA111, and TDP-4315,16. Near-atomic structures are already available for the ordered fibril cores of the LCDs of FUS14 and TDP-4315
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