W0169

Caught in the Act: F-actin Nucleation, Polymerization and Branching Mediated by the Anti-parallel Dimer. Robbie Reutzel¹, Craig Yoshioka¹, Lakshmanan Govindasamy¹, Elena G. Yarmola², Michael R. Bubb², and Robert McKenna¹, ¹Dept. of Biochemistry and Molecular Biology, Univ. of Florida College of Medicine, Gainesville, FL 32610, ²Research Service, Malcom Randall Dept. of Veterans Affairs Medical Center and the Univ. of Florida.

Actin filament nucleation, polymerization and branching are crucial steps in many forms of cell motility and intracellular organelle movement in a wide range of organisms. Previous data suggests that an anti-parallel actin dimer can incorporate itself into growing filamentous actin (F-actin) and may have roles in both nucleation and branching. Furthermore, it is a widespread belief that nucleation is spawned from an actin trimer complex. We present the crystal structure of actin demonstrating newly formed dimer and trimer interactions induced by the dehydration from an orthorhombic crystal system P2₁2₁2₁ into a rearranged tetragonal crystal system P4₃2₁2, with a shrinkage of the unit cell volume of almost 300,000 ų. This transition also induced a more stable crystal lattice, increasing the resolution of diffraction from 3.5 to 2.3 Å resolution. We postulate that this is the first structural evidence for the “sidetrack” polymerization pathway and the first time documented account of a single crystal changing space groups through the loss of solvent. This tetragonal form structure reveals evidence for contacts of actin dimers and trimers hat may have implications to the formation of F-actin.