VlsE, variable major protein-like sequence expressed, is one of many lipoproteins produced by the Lyme disease spirochete, Borrelia burgdorferi . Far-UV circular dichroism (CD) spectroscopy confirms that recombinant, soluble VlsE is predominantly alpha-helical in solution, as also observed in the published crystal structure. CD and tyrosine fluorescence experiments show that the equilibrium and kinetic folding mechanisms for rVlsE are two-state, corresponding to a free energy of unfolding of 20 +/- 5kJ/mol (20°C, pH 7). The low stability for a protein of this size (341 residues) is also clear from midpoints of the unfolding transitions when induced by chemical denaturants GuHCl and urea: 0.4 +/- 0.1M and 1.5 +/- 0.5M, respectively (20°C, pH 7). Thermal unfolding of VlsE shows a transition midpoint of 55 +/- 2°C (5mM phosphate buffer, pH 7). The folding speed of VlsE in buffer (20°C, pH 7) is 7 +/- 2s-1, several orders of magnitude slower than what its native-state topology (contact order) predicts. The alpha-helical content appears to play an important role in the immunodominance of one of VlsE's conserved invariable region, IR6. As expected, recombinant VlsE is found to be structurally similar to the native, membrane-bound protein, according to immunoprecipitation and Western blot analysis. This lends validity to the in vitro findings on recombinant VlsE and their implications for VlsE's function in vivo
