Structure of the 59 kD Armadillo Repeat Region of Beta-Catenin Determined by Mad Phasing. William I. Weis and Andrew H. Huber Dept. of Structural Biology, Stanford University School of Medicine, Stanford, CA, USA

Beta-catenin is a 97 kD cytoplasmic protein that forms an integral part of cellular junctions by linking cadherin adhesion molecules to the cytoskeleton. Beta-catenin and its Drosophila homologue armadillo are also downstream effectors of the wnt/wingless signaling pathway that controls cell fate determination in early embyrogenesis. We have determined the structure of the central 59 kD region of beta-catenin that consists of twelve imperfect sequence repeats known as armadillo repeats. The structure was determined by MAD phasing from the selenomethionyl-substituted protein. Data were measured at SSRL beamline 1-5 at four wavelengths. Structure factor amplitudes of the Se atoms (|Fa|) were extracted using the MADLSQ procedure. A combination of automated Patterson search and difference Fourier methods using the |Fa| values provided 11 of the 16 possible Se sites. Heavy atom refinement using an MIR-like maximum-likelihood algorithm yielded 4 more sites. Experimental phases to 2.4 Angstrom resolution were computed from the 15 site model, with an overall figure-of-merit of 0.65. The resulting electron density map was readily interpretable. Details of the MAD experiment and phasing will be discussed.