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...