Selected prokaryotic actin homologs aligned with eukaryotic actins and actin-related proteins 2 and 3. Green boxes 1 and 2 highlight the major inserts in Arp 2, 3 distinguishing them from actins : a loop in subdomain 4 of Arp3 (green box 1) and elongated loops in subdomain 3 in both Arp 2 and Arp 3 (green box 2). ALP, actin-like protein. Red boxes indicate homologous inserts shared between crenarchaeal and eukaryotic proteins. For the complete legend, see Additional File 5.