The crystal structure of a soluble form of Borrelia burg-dolferi outer surface protein A (OspA) complexed with the Fab fragment of a monoclonal antibody has revealed an unusual structure that has a repetitive antiparallel P topology with a nonglob-ular, single layer P-sheet connecting the globular N-and C-terminal domains. Earlier NMR studies have shown that the local structure of OspA including the single layer P-sheet is similar to the crystal structure. Here we report a small angle X-ray scattering (SAXS) study of the global conformation of OspA in solution. The radius of gyration (R,) and the length distribution function (P (r)) of OspA measured by SAXS in solution are nearly identical to the calculated ones from the crystal structure, respectively. The NMR and SAXS experiments complement each other to show that OspA including the central single-layer P-sheet is a stable structure in solution, and that the OspA crystal structure represents the predominant solution conformation of the protein. A continuing challenge in current structural biology is to identify the relative importance of different factors in protein stability and folding. These factors include the hydrophobic effect, hydrogen bonding, packing, and electrostatic interactions. Studies from a variety of soluble proteins have established that the formation of a hydrophobic core that removes hydrophobic surface from contact with water is a major force for protein folding (Kauzmann, 1959; Tanford, 1980; Dill, 1990). However, the crystal structure of a soluble form of outer surface protein A (OspA, an abundant im-munogenic lipoprotein of Lyme disease spirochete Borrelia burg-dolferi) complexed with the Fab fragment of a monoclonal antibody has revealed an unusual structure (Li et al., 1997). OspA has a repetitive antiparallel P topology with a nonglobular, single layer P-sheet connecting the globular N-and C-terminal domains (see Fig. 1A). Interestingly, this central P-sheet consists largely of polar Reprint requests to:. amino acids and it is solvent-exposed on both sides. Such a solvent-exposed, single-layer P-sheet has never been found previously; most antiparallel P-sheets observed in protein structures are am-phipathic, with the hydrophobic face buried in the interior hydro-phobic core. Considering the well-accepted principles that the formation of the hydrophobic core is the major driving force for protein folding, one may ask if the central P-sheet in OspA is only stabilized in the crystal structure due to antibody binding and/or lattice packing. One might expect that the OspA molecule is bent along the single-layer P-sheet in the free form. To elucidate the molecular basis for the high conformational stability of the unique single-layer P-sheet, it is essential to first establish whether the crystal structure represents the predominant conformation of OspA in solution and thus can be used as a reliable model for the solution structure. In an earlier study, NMR chemical shift analysis suggested that the local structure of OspA in solution, inc...

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