W0169

Shedding Light on New Hematopoietic Cytokines: Structure of the flt3 Ligand. Savvas N. Savvides¹, Tom Boone², and P. Andrew Karplus¹, ¹Program in Biophysics, Cornell University, Ithaca, NY 14853, USA. ²AMGEN, Thousand Oaks, CA 91320-1789, USA

The crystal structure of the new hematopoietic cytokine, flt3 ligand (flt3L), has been determined at 2.2 Å resolution using multiple isomorphous replacement. flt3L acts on early hematopoietic cells (stem cells and progenitor B cells) by binding to and activating its type III tyrosine kinase receptor (flt3R) on the cell surface. This initiates a cascade of intracellular responses leading to cell mobilization and proliferation. From a biomedical perspective, flt3L has already shown substantial promise in the correction of malfunctioning hematopoiesis and in cancer immunotherepy, and is currently in phase I/II clinical trials. The structure of flt3L will be correlated with bioactivity and receptor binding results from structure/function studies, and will facilitate further mutagenesis experiments ultimately leading to an understanding of the requirements for a productive flt3L-flt3R complex.

Our structure reveals the biologically relevant homodimeric form of flt3L with two dimers in the asymmetric unit. The structure solution was initially hampered by poor heavy atom derivatives. Iterative phase improvement with solvent flattening and four-fold noncrystallographic symmetry averaging resulted in an electron density map that allowed tracing of the polypeptide chain.