Dissociation of the light-harvesting membrane protein complex I from Rhodobacter sphaeroides under high hydrostatic pressure

† This paper was presented at the LIIth European High Pressure Research Group (EHPRG 52) Meeting in Lyon (France), 7–12 September 2014.

Abstract

The light-harvesting complex 1 (LH1) from Rhodobacter sphaeroides is an excellent model system for investigating the stability of oligomeric membrane proteins under high hydrostatic pressure. The currently investigated LH1 forms a 16-meric ring structure of B825 subunits. B825 is a heterodimer of transmembrane α- and β-polypeptide chains, which non-covalently binds two bacteriochlorophyll a molecules. These pigment molecules were used as intrinsic spectroscopic sensors to follow the dissociation reaction. Our results demonstrate that the LH1 dissociates into B825 subunits through an intermediary tetrameric unit B845. The dissociation mechanism depends on pressure. At ∼200–500 MPa the dissociation corresponds to a pseudo-first-order reaction, characterised by the apparent reaction rate at atmospheric pressure k₀ = 3·10⁻⁵ s⁻¹, activation volume ΔV^‡ = −4 mL/mol, and free energy of activation ΔG^‡ = 26 kJ/mol. Below 200 MPa and above 500 MPa, the reaction is more complex, including further dissociation of B825 into monomers B777.

Keywords:

• integral membrane protein
• high pressure
• protein dissociation

Acknowledgements

The authors would like to thank C. N. Hunter and his research group from the University of Sheffield for the preparation and purification of the LH1 sample, and M. Rätsep, K. Leiger and H. Salujärv for their expert technical assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

† This paper was presented at the LIIth European High Pressure Research Group (EHPRG 52) Meeting in Lyon (France), 7–12 September 2014.

Additional information

Funding

This work was supported by the Estonian Research Council under [Grant IUT02-28].