PROJECT SUMMARY/ABSTRACT Non-muscle myosin II (NMII) contractility is critical to number cellular processes from development to disease. NMII is an ATP-dependent molecular motor that functions as a dimer composed of two heavy chains, which are made of an ATPase/motor domain that binds actin and a coiled-coil tail domain. It is also bound to two accessory proteins, the essential light chain which plays structural roles, and the regulatory light chain which is the target of phosphorylation, integrating the molecule into a myriad of signaling pathways. Phosphorylation of the regulatory light chain leads to a relief of an autoinhibition, opening up the molecule and making it competent to bind actin, however, by itself it is a poor motor protein. The second required step of activation is oligomerization into higher ordered, bi-polar filaments. This oligomerization is thought to be regulated by the tail domain where again, phosphorylation is hypothesized to be the main driver of this transition. While decades of research have revealed much about these biochemical and biophysical properties, we questioned whether there were other, yet-to-be revealed, mechanisms that may contribute to NMII’s regulation. The overarching goal of this proposal is to understand the mechanisms that regulate NMII contractility. In Project 1, we explore a potential novel NMII binding protein, Split Discs (Spdi). Spdi’s human homolog, SPECC11L has been implicated in a spectrum of cranial-facial pathologies, highly suggestive of abearent cranial neural crest cell migration. SPECC1L was initially characterized as actin-microtubule crosslinking proteins, however data from my lab suggests that its target is NMII and actin. It is our hypothesis that Spdi binds NMII to regulate its contractility. Through a series of biochemical characterization and cell biology experiments where we elucidate the mechanism by which Spdi associates with NMII, and employ an ex-vivo developmental model to understand how its