High Pressure Research, 2015 Vol. 35, No. 2, 176–180, http://dx.doi.org/10.1080/08957959.2015.1017817 Dissociation of the light-harvesting membrane protein complex I from Rhodobacter sphaeroides under high hydrostatic pressure † Marit Puusepp a , Liina Kangur a and Arvi Freiberg a , b∗ a Institute of Physics, University of Tartu, Tartu, Estonia; b Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia (Received 5 September 2014; final version received 17 January 2015) 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 transmem- brane 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 corre- sponds to a pseudo-first-order reaction, characterised by the apparent reaction rate at atmospheric pressure k 0 = 3·10 -5 s -1 , activation volume V ‡ =- 4 mL/mol, and free energy of activation G ‡ = 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 Introduction The aim of the current paper is to investigate hydrostatic pressure effects on the core light- harvesting complex I (LH1) from Rhodobacter (Rb.) sphaeroides. Pressure effects on the bacterial light-harvesting pigment–protein complexes were first studied by Freiberg et al.[1]. The LH1 is an excellent model system for investigating the stability of oligomeric membrane proteins under high hydrostatic pressure. In wild-type organisms the LH1 complex is S-shaped,[2] consisting of 28 identical het- erodimeric subunits,[3] which are conveniently named B825 after their peak absorption wavelength. The currently investigated mutant LH1 forms a closed ring and consists of 16 het- erodimeric subunits. However, it has been shown that the LH1 complex may vary in size, taking arc-, polygon-, ellipse-, etc. like shapes.[4] The intact LH1 ring has an absorption peak at 875 nm, hence its spectroscopic indication as B875. The B825 subunit in Rb. sphaeroides corresponds to *Corresponding author. Email: arvi.freiberg@ut.ee † This paper was presented at the LIIth European High Pressure Research Group (EHPRG 52) Meeting in Lyon (France), 7–12 September 2014. © 2015 Taylor & Francis