Molecular determinants of Arc oligomerization and formation of virus-like capsids
Authors: Eriksen Maria Steene , Nikolaienko Oleksii , Hallin Erik Ingmar , Grødem Sverre , Bustad Helene J. , Flydal Marte Innselset , O’Connell Rory , Hosokawa Tomohisa , Lascu Daniela , Akerkar Shreeram , Cuéllar Jorge , Chambers James J. , Merski Ian , Muruganandam Gopinath , Loris Remy , Kanhema Tambudzai , Hayashi Yasunori , Stratton Margaret M. , Valpuesta José M. , Kursula Petri , Martinez Aurora , Bramham Clive R. 
Published: June 2019

Expression of activity-regulated cytoskeleton-associated protein (Arc) is critical for long-term synaptic plasticity, memory formation, and cognitive flexibility. The ability of Arc to self-associate and form virus-like capsid structures implies functionally distinct oligomeric states. However, the molecular mechanism of Arc oligomerization is unknown. Here, we identified a 28-amino-acid region necessary and sufficient for Arc oligomerization. This oligomerization region is located within the second coil of a predicted anti-parallel coiled-coil in the N-terminal domain (NTD). Using alanine scanning mutagenesis, we found a 7-amino-acid motif critical for oligomerization and Arc-mediated transferrin endocytosis in HEK cells. Intermolecular fluorescence lifetime imaging in hippocampal neurons confirmed self-association mediated by the motif. To quantify oligomeric size, we performed a single-molecule photobleaching analysis of purified Arc wild-type and mutant. This analysis revealed a critical role for the NTD motif in the formation of higher-order Arc oligomers (30-170 molecules). Moreover, assembly of higher-order wild-type Arc oligomers was significantly enhanced by addition of GFP RNA. Purified wild-type Arc formed virus-like capsids, as visualized by negative-stain EM, and was estimated by light scattering analysis to contain 40-55 Arc units. In contrast, mutant Arc formed a homogenous dimer population as demonstrated by single-molecule TIRF imaging, size-exclusion chromatography with multi-angle light scattering analysis, small-angle X-ray scattering analysis, and single-particle 3D EM reconstruction. Thus, the dimer appears to be the basic building block for assembly. Herein, we show that the NTD motif is essential for higher-order Arc oligomerization, assembly of virus-like capsid particles, and facilitation of oligomerization by exogenous RNA.

SIGNIFICANCE

Arc protein is rapidly expressed in neurons in response to synaptic activity and plays critical roles in synaptic plasticity, postnatal cortical developmental, and memory. Arc has diverse molecular functions, which may be related to distinct oligomeric states of the protein. Arc has homology to retroviral Gag protein and self-assembles into retrovirus-like capsid structures that are capable of intercellular transfer of RNA. Here, we identified a motif in the N-terminal coiled-coil domain of mammalian Arc that mediates higher-order oligomerization and formation of virus-like capsids. The basic building block is the Arc dimer and exogenous RNA facilitates further assembly. The identified molecular determinants of Arc oligomerization will help to elucidate the functional modalities of Arc in the mammalian brain.