Relating structure, biomechanics and function of single membrane proteins
Sonia Antoranz Contera1, Kislon Voïtchovsky1, Hayato Yamashita2, Takayuki Uchihashi2,Toshio Ando2, J.F. Ryan1
1

Bionanotechnology IRC, Clarendon Laboratory, Physics Department, University of Oxford,
Oxford OX4 3AP, UK
2
Graduate School of Natural Science & Technology, Kanazawa University, Kakuma,Kanazawa, Ishikawa 920-1192, Japan
E-mail: s.antoranzcontera1@physics.ox.ac.uk

One of the greatest challenges of biophysics is to understand how membrane protein
sequences relate to the local forces that dynamically control their structure and drive theirfunction. AFM is one of the most powerful techniques to address this problem, however, mostof the single membrane protein unfolding AFM studies available fail to relate the potential
barriers observed to specific molecular interactions relevant for the protein function.
In some of our recent papers, we have combined membrane indentation with AFM1, AC-force
spectroscopy2, AFM unfolding and extraction of model peptides inserted planar lipid bilayers
at different speeds3, unfolding and extraction of bacteriorhodopsin (bR) from purple
membranes4 and comparison of pulling experiments with molecular dynamics simulations3
to relate structure, nano-biomechanics, local hydration, mechanical properties and function of
membrane proteins.
In particular, retinal proteins convert the energy of a single photon into large structuralchanges subsequently used to carry out various tasks. This is achieved by a complex
combination of local dynamical interactions controlling the protein biomechanics, allowing
efficient amplification of the retinal isomerization. In the case of the retinal containing protonpump bR we have shown that steric, specific interactions create a rigid scaffold in the protein
extracellular region4. This scaffold, which encloses the retinal, controls bR local
biomechanical properties and anchors the protein into the membrane. In contrast, thecytoplasmic side of bR is mainly governed by relatively weak non-specific electrostaticinteractions which provide the flexibility necessary for large cytoplasmic structural
rearrangements during the photocycle. Finally we show that bR mechanical properties are partof the strategy adopted by bR to efficiently function in extreme halophilic environments.
References
1. Voitchovsky, K., Antoranz Contera, S., Kamihira, M., Watts, A. & Ryan, J. F. Differential stiffness and lipid
mobility in the leaflets of purple membranes. Biophys. J. 90, 2075-2085 (2006).
2. Antoranz Contera, S., Voïtchovsky, K., Yamashita, H., Uchihashi, T, Ando, T., Ryan J.F. Small amplitude
sub-molecular resolution AC-AFM and High-Speed AC-AFM of purple membranes in fluid: non-continuum
effects inside the double-layer. Biophys. J. Suppl. Meeting Abstract (2006).
3. Antoranz Contera, S., Lemaître, V., De Planque, M.R.R., Watts, A., Ryan, J.F. Unfolding and extraction of atransmembrane α-helical peptide: Dynamic force spectroscopy and molecular dynamics simulations. Biophys. J.,
89 , 3129-3140 (2005).
4. Voitchovsky, K., Antoranz Contera, S. & Ryan, J. F. “Single-molecule nano-biomechanics of rhodopsin-likeproteins”:. Biophys. J. (2007) submitted.