The love-hate relationship between APC2 and Diaphanous: Dissecting the mechanism of APC2-Diaphanous dependent actin assembly in the Drosophila syncytial embryo. Ezgi Kunttas-Tatli¹, Rebecca Webb², Orr Rozov¹, Kelly Shibuya¹, Brooke M. McCartney¹. 1) Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA; 2) Biology Department, University of Pittsburgh at Johnstown, Johnstown, PA.

   Cell division, cell shape change and cell migration are a few of the many cellular processes that require precise cytoskeletal rearrangements mediated by an orchestra of actin and microtubule-associated proteins. The colon cancer tumor suppressor Adenomatous polyposis coli (APC), a well-known component of the Wnt signaling pathway, is a poorly understood regulator of the actin cytoskeleton. The Drosophila syncytial embryo is a powerful in vivo model system to study the role of APC in actin reorganization. The early embryo undergoes synchronous nuclear divisions without cytokinesis, and the actin cytoskeleton undergoes dynamic changes coordinated with the cell cycle. During interphase, actin is organized in caps above each nucleus, and in metaphase actin is reorganized into pseudocleavage furrows that act as physical barriers to ensure mitotic fidelity. A complex of APC2 and the formin Diaphanous (Dia) regulates the formation and extension of the furrows. Mapping of the binding domains revealed that APC2 interacts with Dia via the 20 amino acid repeats (20Rs) and deletion of these domains results in furrow extension defects. Because the 20Rs are highly phosphorylated, we predicted that phosphorylation of the 20Rs may play a role in the APC2-Dia interaction. Consistent with that hypothesis, targeted mutants that disrupt or mimic phosphorylation both disrupt furrow extension. While our original in vivo analysis suggested a collaborative interaction between APC2 and Dia, in vitro analysis revealed that the APC2-20Rs potently inhibit Dias actin assembly activity. To reconcile these findings, we are investigating the relationship between APC2 and Dia by live analysis of actin dynamics and actin filament turnover throughout the actin cycle in the syncytial embryo.