Spectroscopic Characterization of Two Soluble Transducers from the Archaeon Halobacterium salinarum

In the present study, structural aspects of the two soluble transducers, HtrX and HtrXI, from the archaeon H. salinarum have been examined using UV circular dichroism and steady-state fluorescence spectroscopies. Circular dichroism (CD) data indicate that both HtrX and HtrXI exhibit salt-dependent protein folding. Under low-ionic-strength conditions (0.2 M NaCl or KCl) the CD spectra of HtrXI is similar to that of the Gdn-HCl- or urea-denatured forms and is indicative of random coil structure. In contrast, the CD spectrum of HtrX under low-ionic-strength conditions contains roughly 85% α-helical character, indicating a significant degree of folding. Addition of NaCl or KCl to solutions of HtrX or HtrXI results in CD features consistent with predominately α-helical character (>95%) for both proteins. In addition, the transition points (i.e., ionic strengths at which the protein converts from random coil to α-helical character) are quite distinct and dependent upon the type of salt present (i.e., either NaCl or KCl). Accessibility of tryptophan residues to the solvent was also examined for both HtrX and HtrXI in both folded and unfolded states using Kl quenching. The Stern–Volmer constants obtained suggest that the tryptophans (Trp35 in HtrX and both Trp47 and Trp74 in HtrXI) are partially exposed to the solvent, indicating that they are located near the surface of the protein in all three cases. Furthermore, fluorescence quenching with the single Trp mutants Trp74AIa and Trp47AIa of HtrXI indicates different environments for these two residues.