This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We are currently pursuing an innovative crystallography project to examine the structural basis of actin filament formation using novel x-ray crystallographic techniques. Crystals thought to contain actin filaments form modulated protein crystals when exposed to conditions known to promote actin filament formation. Profilin:actin (PA) crystals can be transformed into a modulated state which is characterized by unusual off-lattice satellite reflections. Profilin:Actin crystals were obtained and a complete data set to 3 [unreadable] was collected. The diffraction pattern showed distinct first-order satellite reflections that are indicative of actin filament formation in situ. The satellite reflections are described by the addition of one q-vector, which refined to q1 = 0.00a* + 0.29b* + 0.00c. Conditions for the collection of modulated PA have been optimized, which includes cryocooling conditions. The next step in this project involves the collection of high-resolution diffraction data.