Protein conformational changes in the bacteriorhodopsin photocycle

Sriram Subramaniam, Martin Lindahl, Per Bullough, A. R. Faruqi, Jörg Tittor, Dieter Oesterhelt, Leonid Brown, Janos Lanyi, Richard Henderson

Research output: Contribution to journalArticlepeer-review

Abstract

We report a comprehensive electron crystallographic analysis of conformational changes in the photocycle of wild-type bacteriorhodopsin and in a variety of mutant proteins with kinetic defects in the photocycle. Specific intermediates that accumulate in the late stages of the photocycle of wildtype bacteriorhodopsin, the single mutants D38R, D96N, D96G, T46V, L93A and F219L, and the triple mutant D96G/F171C/F219L were trapped by freezing two-dimensional crystals in liquid ethane at varying times after illumination with a light flash. Electron diffraction patterns recorded from these crystals were used to construct projection difference Fourier maps at 3.5 Å resolution to define light-driven changes in protein conformation. Our experiments demonstrate that in wild-type bacteriorhodopsin, a large protein conformational change occurs within ~ 1 ms after illumination. Analysis of structural changes in wild-type and mutant bacteriorhodopsin, under conditions when either the M or the N intermediate is preferentially accumulated reveals that there are only small differences in structure between M and N intermediates trapped in the same protein. However, a considerably larger variation is observed when the same optical intermediate is trapped in different mutants. In some of the mutants, a partial conformational change is present even prior to illumination, with additional changes occurring upon illumination. Selected mutations, such as those in the D96G/F171C/F219L triple mutant, can sufficiently destabilize the wild-type structure to generate almost the full extent of the conformation change in the dark, with minimal additional light-induced changes. We conclude that the differences in structural changes observed in mutants that display long-lived M, N or O intermediates are best described as variations of one fundamental type of conformational change, rather than representing structural changes that are unique to the optical intermediate that is accumulated. Our observations thus support a simplified view of the photocycle of wild-type bacteriorhodopsin in which the structures of the initial state and the early intermediates (K, L and M1) are well approximated by one protein conformation, while the structures of the later intermediates (M2, N and O) are well approximated by the other protein conformation. We propose that in wild-type bacteriorhodopsin and in most mutants, this conformational change between the M1 and M2 states is likely to make an important contribution towards efficiently switching proton accessibility of the Schiff base from the extracellular side to the cytoplasmic side of the membrane.

Original languageEnglish (US)
Pages (from-to)145-161
Number of pages17
JournalJournal of Molecular Biology
Volume287
Issue number1
DOIs
StatePublished - Mar 19 1999
Externally publishedYes

Keywords

  • Conformational change
  • Electron crystallography
  • Proton pump
  • Seven-helix membrane protein
  • Trapped intermediates

ASJC Scopus subject areas

  • Virology

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