Structure of D. melanogaster ARC1 reveals a repurposed molecule with characteristics of retroviral Gag

Abstract

18The tetrapod neuronal protein ARC and its D. melanogaster homologue, dARC1, have 19 important but differing roles in neuronal development. Both are thought to originate 20 through exaptation of ancient Ty3/Gypsy retrotransposon Gag genes, with their novel 21 function relying on an original capacity for self-assembly and encapsidation of nucleic 22 Keywords: ARC; Gag; CA; Neuronal development; TY3/Gypsy. 33 34 lineages of Ty3/Gypsy, since dARC1 clustered with insect Ty3/Gypsy retrotransposons 60 and tetrapod ARCs clustered with fish Ty3/Gypsy retrotransposons (15). 61The relevance of ARC's retrotransposon origin to its function in synaptic plasticity 62 was not immediately obvious until the recent observation that mam-ARC and dARC1 can 63 self-assemble into particles and package RNA for potential transfer between cells (12, 15), 64 similarly to retrotransposons and retroviruses (18, 19). In D. melanogaster, it is proposed 65 that dARC1 expressed at neuromuscular junction presynaptic boutons assembles into 66 particles that encapsidate dARC1 mRNA. Loaded particles might then be packaged and 67 released as extracellular vesicles for intercellular transfer to the post synapse where 68 mRNA release and translation can take place (12, 15), Similarly, mam-ARC can also 69 encapsidate ARC mRNA into particles, allowing transfer from donor to recipient neurons, 70where ARC mRNA can be translated (15). 71Since both dARC1 and mam-ARC are able to form capsid-like particles (12, 15), it 72 seems likely that they share a degree of structurally similarity. To date, crystal structures of 73 the individual domains from rARC have been determined (16) along with the solution NMR 74 structure of the rARC CA (20). Here we report two crystal structures of the entire CA 75 region of dARC1 at 1.7 Å and 2.3 Å and consider these structures in comparison to those 76 of rARC and retroviral CA. dARC1 comprises two α-helical domains with a fold related to 77 that observed in the CA-NtD and CA-CtD of orthoretroviral and spumaretroviral CA. 78However, we observe significant divergence in the NtD of dARC1 where an extended 79 hydrophobic strand that packs against α1 and α3 of the core fold replaces the N-terminal 80 β-hairpin and helix α1 found in orthoretroviral CAs. In the rARC structure, this hydrophobic 81 strand is replaced by peptides from the binding partners Ca 2+ /calmodulin-dependent 82 protein kinase 2A (CamK2A) and transmembrane AMPAR regulatory protein γ2 (TARPγ2) 83 and may represent a functional adaptation for the recruitment of partner proteins. We also 84show that dARC1 utilises the same CtD-CtD interface required for assembly of retroviral 85 7 CA into mature particles and propose that this obligate dimer represents a building block 86 for dARC1 particle assembly. Further examination of the relationship between dARC1, 87 mam-ARC and Gag from Ty retrotransposon families, reveals that, although dARC1 and 88 mam-ARC are functional orthologues, the structural divergence in dARC1 and mam-ARC 89 CA domains is consistent with the noti...

Molecular physiology of Arc/Arg3.1: The oligomeric state hypothesis of synaptic plasticity

“…111 Interestingly, the mammalian Arc N-lobe in complex with ligand adopts a βstrand structure similar to dArc N-lobe. 78,111 As discussed in Hallin et al, 86 this raises the possibility that dArc N-lobe could bind peptide ligands under conditions that liberate the N-terminal strand, albeit with a binding specificity different from that of mammalian Arc N-lobe. 111 Another recent study performed structural characterization of the dArc lobe domains.…”

“…81,110 A high-resolution crystal structure of the dArc1 CA reveals a homodimer and a dimerization interface in the C-lobe homologous to retroviral CA. 78 In another major advance, single particle cryo-EM was used to resolve the structure of dArc capsids. 110 The dArc proteins both assemble into icosahedral capsids, with the capsid shell formed by 240 copies of the dArc CA.…”

“…Drosophila has two Arc proteins, dArc1 and dArc2, encoded by separate genes. The mammalian and dArc proteins independently evolved from the domestication of different Ty3‐Gypsy retrotransposons, resulting in proteins with distinct structures 78 . dArcs have a bilobar CA with N‐lobe and C‐lobe.…”

“…The work suggested that Arc protein and modern retroviruses originated from ancient Ty3/Gypsy retrotransposon elements. Current evidence indicates that the Arc gene has been repurposed in separate evolutionary events towards mammalian Arc and Drosophila , which has two Arc genes (dArc1 and dArc2) 78–81 . In an exciting new development, Arc protein from mammals and Drosophila were shown to self‐assemble into retrovirus‐like capsid structures that contain Arc mRNA.…”

Arc1 and the microbiota together modulate growth and metabolic traits inDrosophila

“…Accordingly, reduced Arc expression impairs learning ability in rodents (Guzowski et al, 2000; Shandilya and Gautam, 2020), and defects in human Arc have been linked to neurological and neurodevelopmental disorders, including Alzheimer’s disease (Bi et al, 2018), autism spectrum disorders (Alhowikan, 2016), and schizophrenia (Fromer et al, 2014). Both mammalian Arc and fly Arc1 encode retroviral group-specific antigen-like amino acid sequences, and are predicted to have independently derived from ancient Ty3/Gypsy retrotransposons (Ashley et al, 2018; Campillos et al, 2006; Cottee et al, 2019; Pastuzyn et al, 2018). Recently, it was shown that Arc/Arc1 can self-assemble into capsid-like structures that package and transport mRNAs into cultured neuronal cell lines and across synapses in vivo , constituting a novel mechanism of cell-cell communication (Ashley et al, 2018; Erlendsson et al, 2019; Pastuzyn et al, 2018).…”