A cycle of actin polymerization/disassembly begins in Step 1 when PCARE recruits an actin nucleation promoting factor, likely Wasf3, to the site of disc morphogenesis. This actin nucleation promoting factor associates with the Arp2/3 complex to nucleate branched actin filaments (F-actin). In Step 2, continued actin polymerization pushes the ciliary membrane outwards. The Arp2/3 complex remains as a structural component of these filaments. In Step 3, while the membrane remains protruded, actin filaments are disassembled and retracted allowing the evaginated membrane to flatten. In Step 4, actin has fully retracted and the initiation step of disc formation is complete. Meanwhile, the newly forming disc continues to elongate through ongoing addition of membrane material. The cartoon is updated from [44]. See also Movie S1 to view the entire process of disc formation.