CRYSTAL STRUCTURE OF THE C DOMAIN OF SYNAPSIN IA FROM BOVINE. Lothar Esser¹, Chyung-Ru Wang², Cynthia Smagula¹, Thomas Südhof¹, and Johann Deisenhofer^*1. ¹Howard Hughes Medical Institute Dallas, TX 75235; ²Gwen Knapp Center for Lupus and Immunology Research, The University of Chicago, IL 60637

Synapsin Ia is a neuronal protein whose ability to bind to both synaptic vesicles and actin filaments depends on its phosphorylation state. Thus, a regulatory role in the process of synaptic transmission has been ascribed to it. Synapsin Ia is made up of five domains (A - E). Of these, the central hydrophobic domain C (a. a. 113-420 ) is the most conserved region among different species and contains binding sites for synaptic vesicle membranes and actin. Recombinant DNA technology was used to express a 36 kDa fragment of bovine Synapsin Ia comprising residues 110 through 420 ( which we refer to in the following text as SynA ). SynA was also expressed in form of a seleno-methionine variant. SynA crystallizes in the space group P3221 with two molecules per asymmetric unit. The cell dimensions of the seleno-methionine variant are a = b = 76.39 Å, c = 180.94 Å, [[alpha]] = [[beta]] = 90deg., [[gamma]] = 120deg. at 150 K. A mercury derivative allowed phase determination by SIR to 2.8 Å resolution. However, phases were greatly improved and extended to 2.4 Å by MIR with optimized anomalous scattering techniques based on four data sets from two crystals. Data sets at selected wavelengths of a seleno-methionine SynA crystal and that of a mercury derivative of wild-type SynA, were collected at the beamline X4A in Brookhaven. The high quality of the electron density in ordered regions allowed an unambiguous assignment of the sequence. The current model consists of 286 amino acids (92%) and contains 18 seleno-methionine residues as well as 146 water molecules. The crystallographic R-factors for the current model are 28.9% (free) and 22.5% (work) for data in the resolution range between 6.0 Å and 2.4 Å. SynA can be classified as an alpha-beta structure. Further refinement and structure analysis is in progress.