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Biophysical Journal 70: 2358-2365 (1996)
© 1996 the Biophysical Society

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Reversal of the surface charge asymmetry in purple membrane due to single amino acid substitutions.

K C Hsu, G W Rayfield and R Needleman

Physics Department, University of Oregon, Eugene 97403, USA.

ABSTRACT

Twenty-seven mutant bacteriorhodopsin's were screened to determine the PKa for reversal of the permanent electric dipole moment. The photoelectric response of an aqueous purple-membrane suspension was used to determine the direction of the purple-membrane dipole moment as a function of pH. The pK(a) for the dipole reversal of wild-type bacteriorhodopsin is 4.5. Six of the 27 mutant bacteriorhodopsin's were found to have a pK(a) for dipole reversal larger than that of wild-type bacteriorhodopsin. Two of these mutants, L93T and L93W, involve a neutral amino acid substitution in the interior of the protein. The direction of the purple-membrane permanent electric dipole moment is determined by the purple-membrane surface charge asymmetry. We conclude that these two substitutions, which do not involve charge replacement, alter the pK(a) for the reversal of the purple-membrane surface charge asymmetry. We suggest that these changes to the pK(a) are due to altered protein folding at the surface of the purple-membrane induced by single-site substitutions in the protein interior.




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J. Xu, A. B. Stickrath, P. Bhattacharya, J. Nees, G. Varo, J. R. Hillebrecht, L. Ren, and R. R. Birge
Direct Measurement of the Photoelectric Response Time of Bacteriorhodopsin via Electro-Optic Sampling
Biophys. J., August 1, 2003; 85(2): 1128 - 1134.
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Copyright © 1996 by the Biophysical Society.