Figure 1. Rat and Drosophila Arc proteins form virus-like capsids via a conserved retroviral Gag capsid region.

A. Representative negative stain EM images of purified rat Arc protein (prArc). Inset shows magnified Arc capsids (from [5]). B. Atomic resolution model of a dArc1 capsid derived from cryo-EM data as reported in [17] (image courtesy of Simon Erlendsson). C. Phylogeny based on an amino acid alignment of tetrapod Arc, fly dArc1, and Gag sequences from related Ty3/gypsy retrotransposons. In lineages without Arc genes, the most closely related sequences to Arc are Gag-pol polyproteins flanked by long terminal repeats (LTRs) as expected in bona fide Ty3/gypsy retrotransposons (from [5]).