Spire stimulates nucleation by Cappuccino and binds both ends of actin filaments

Abstract

An actin mesh fills both mouse and fly oocytes. The meshes are built by a conserved mechanism and used to establish polarity. Two actin nucleators, Spire and Cappuccino, collaborate to build actin filaments that connect vesicles and the cortex. Direct interaction between Spire and Cappuccino is required for in vitro synergistic actin assembly; however, we understand little about why the interaction is necessary. To mimic the geometry of Spire and Cappuccino in vivo, we immobilized Spire on beads. We found that increased nucleation is a major part of synergy and that Spire alone binds both barbed- and pointed-ends of actin filaments. We identified Spires barbed-end binding domain. Partial rescue of fertility by a loss-of-function mutant indicates that barbed-end binding is not necessary for Spires in vivo function, but that it may play a role under normal circumstances. We propose that Spire stimulates nucleation by Cappuccino in a manner similar to the collaboration between APC and mDia1.\n\nSummaryActin nucleators Cappuccino and Spire collaborate to build an actin mesh in oocytes. Data demonstrate that the collaboration leads to synergistic actin nucleation, as opposed to elongation. Further, Spire binds both ends of polar, actin filaments, resolving a long-outstanding question.