Proton channel hydration and dynamics of a bacteriorhodopsin triple mutant with an M-state-like conformation

Curvature of purple membranes comprising permanently wedge-shaped bacteriorhodopsin molecules is regulated by lipid content

Purple membrane (PM) from Halobacterium salinarum has been studied by many groups and is commonly described as a flat 2-D crystalline membrane microdomain which contains a hexagonal crystalline lattice of bacteriorhodopsin (BR) trimers in a stoichiometric ratio of 10:1 between lipids and BR. BR is the key protein in the halobacterial photosynthetic system and acts as a light-driven proton pump. Upon absorption of a photon, BR undergoes a cyclic series of intramolecular changes, among them a transient "wedge-like" geometrical change of the protein due to a tilt in helix F, one of the seven alpha-helical domains of BR. Due to the strong coupling between the BRs in the crystalline lattice, this may affect membrane topography. In nature, only low light levels occur and the total number of BRs in the "wedge-shaped" state is negligible. For mutated PMs like PM-D85T and PM-D85N (PM-D85X, X = neutral residue), the change of the membrane topography can be triggered in a pH-dependent manner. PMs containing BR-D85X look like "cups" at certain pH values. How does nature deal with a mutated PM like PM-D96G/F171C/F219L (PM-Tri) which comprises permanently "wedge-shaped" BRs and how does this influence membrane assembly? Astonishingly, we observed that PM-Tri is flat. Obviously, the morphology of Halobacterium salinarum is highly conserved and requires flat PMs to be assembled. We found that the lipid content of PM-Tri is specifically altered to assemble a hexagonal crystalline PM-Tri lattice of flat topography.

Time-resolved quasielastic neutron scattering studies of native photosystems

The internal molecular dynamics of proteins plays an important role in a number of functional processes in native photosystems. Prominent examples include the photocycle of bacteriorhodopsin and electron transfer in the reaction center of plant photosystem II. In this regard, the recently developed technique of time-resolved quasielastic neutron scattering with laser excitation opens up new perspectives for the study of protein/membrane dynamics in specific functional states of even complex systems. The first direct observation of a functionally modulated protein dynamics has just recently been reported for the model system bacteriorhodopsin (Pieper et al., Phys. Rev. Lett. 100, 2008, 228103.), where a transient softening of the protein was observed on a timescale of approximately 1 ms along with the large-scale structural change in the M-intermediate of bacteriorhodopsin. In contrast, photosystem II membrane fragments with inhibited electron transfer show a suppression of protein dynamics approximately 160 mus after the actinic laser flash (Pieper and Renger, Biochemistry 48, 2009, 6111). This effect may reflect aggregation-like conformational changes capable of dissipation of excess excitation energy to prevent photodamage in the absence of Q(A)-->Q(B) electron transfer. These findings indicate that proteins exhibit a remarkable flexibility to accommodate different functional processes. This contribution will discuss methodical aspects, challenges, and recent applications of laser-excited, time-resolved quasielastic neutron scattering.

Transient protein softening during the working cycle of a molecular machine

Proper functioning of proteins usually requires a certain internal flexibility provided by stochastic structural fluctuations on the picosecond time scale. In contrast with conventional steady-state experiments, we report on a novel type of (laser-neutron) pump-probe experiment combining in situ activation of protein function with a time-dependent test of protein dynamics using quasielastic neutron scattering. A "transient protein softening" is shown to occur during the photocycle of bacteriorhodopsin as a direct proof for the functional significance of protein flexibility.

Dynamical heterogeneity of specific amino acids in bacteriorhodopsin

Components of biological macromolecules, complexes and membranes are animated by motions occurring over a wide range of time and length scales, the synergy of which is at the basis of biological activity. Understanding biological function thus requires a detailed analysis of the underlying dynamical heterogeneity. Neutron scattering, using specific isotope labeling, and molecular dynamics simulations were combined in order to study the dynamics of specific amino acid types in bacteriorhodopsin within the purple membrane (PM) of Halobacterium salinarum. Motions of leucine, isoleucine and tyrosine residues on the pico- to nanosecond time scale were examined separately as a function of temperature from 20 to 300 K. The dynamics of the three residue types displayed different temperature dependence: isoleucine residues have larger displacements compared to the global PM above 120 K; leucine residues have displacements similar to that of PM in the entire temperature range studied; and tyrosine residues have displacements smaller than that of the average membrane in an intermediate temperature range. Experimental features were mostly well reproduced by molecular dynamics simulations performed at five temperatures, which allowed the dynamical characterisation of the amino acids under study as a function of local environment. The resulting dynamical map of bacteriorhodopsin revealed that movements of a specific residue are determined by both its environment and its residue type.

Hydration dependence of active core fluctuations in bacteriorhodopsin

We used neutron scattering and specific hydrogen-deuterium labeling to investigate the thermal dynamics of isotope-labeled amino acids and retinal, predominantly in the active core and extracellular moiety of bacteriorhodopsin (BR) in the purple membrane and the dynamical response to hydration. Measurements on two neutron spectrometers allowed two populations of motions to be characterized. The lower amplitude motions were found to be the same for both the labeled amino acids and retinal of BR and the global membrane. The larger amplitude dynamics of the labeled part, however, were found to be more resilient than the average membrane, suggesting their functional importance. The response to hydration was characterized, showing that the labeled part of BR is not shielded from hydration effects. The results suggest that the inhibition of high-amplitude motions by lowering hydration may play a key role in the slowing down of the photocycle and the proton pumping activity of BR.