ABSTRACT Bacteriorhodopsin (BR) is an integral membrane protein, which functions as a light-driven proton pump in Halobacterium salinarum. We report evidence that one or more methionine residues undergo a structural change during the BR->M portion of the BR photocycle. Selenomethionine was incorporated into BR using a cell-free protein translation system containing an amino acid mixture with selenomethionine substituted for methionine. BR->M FTIR difference spectra recorded for unlabeled and selenomethionine-labeled cell-free expressed BR closely resemble the spectra of in vivo expressed BR. However, reproducible changes occur in two regions near 1284 and 900 cm^sup -1^ due to selenomethionine incorporation. Isotope labeled tyrosine was also co-incorporated with selenomethionine in order to confirm these assignments. Based on recent x-ray crystallographic studies, likely methionines which give rise to the FTIR difference bands are Met-118 and Met-145, which are located inside the retinal binding pocket and in a position to constrain the motion of retinal during photoisomerization. The assignment of methionine bands in the FTIR difference spectrum of BR provides a means to study methionine-chromophore interaction under physiological conditions. More generally, combining cell-free incorporations of selenomethionine into proteins with FTIR difference spectroscopy provides a useful method for investigating the role of methionines in protein structure and function.
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Vladislav Bergo, Sergey Mamaev, Jerzy Olejnik, and Kenneth J. Rothschild
Physics Deptartment and Molecular Biophysics Laboratory, Boston University, Boston, Massachusetts 02215
Submitted June 18, 2002, and accepted for publication October 8, 2002.
Address reprint requests to Kenneth J. Rothschild, Physics Dept. and Molecular Biophysics Laboratory, 590 Commonwealth Ave., Boston, MA 02215. Tel.: 617-353-2603; Fax: 617-353-5167; E-mail: kjr@bu.edu.
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