Probing the local lipid environment of the Rhodobacter sphaeroides cytochrome bc(1) and Synechocystis sp PCC 6803 cytochrome b(6)f complexes with styrene maleic acid

Intracytoplasmic vesicles (chromatophores) in the photosynthetic bacteriumRhodobacter sphaeroidesrepresent aminimal structural and functional unit for absorbing photons and utilising their energy for the generation ofATP. The cytochromebc1complex (cytbc1) is one of the four major components of the chromatophore alongsidethe reaction centre-light harvesting 1-PufX core complex (RC-LH1-PufX), the light-harvesting 2 complex (LH2),and ATP synthase. Although the membrane organisation of these complexes is known, their local lipid en-vironments have not been investigated. Here we utilise poly(styrene-alt-maleic acid) (SMA) co-polymers as a toolto simultaneously determine the local lipid environments of the RC-LH1-PufX, LH2 and cytbc1complexes. SMAhas previously been reported to effectively solubilise complexes in lipid-rich membrane regions whilst leavinglipid-poor ordered protein arrays intact. Here we show that SMA solubilises cytbc1complexes with an efficiencyof nearly 70%, whereas solubilisation of RC-LH1-PufX and LH2 was only 10% and 22% respectively. This highsusceptibility of cytbc1to SMA solubilisation is consistent with this complex residing in a locally lipid-richregion. SMA solubilised cytbc1complexes retain their native dimeric structure and co-purify with 56 ± 6phospholipids from the chromatophore membrane. We extended this approach to the model cyanobacteriumSynechocystissp. PCC 6803, and show that the cytochromeb6fcomplex (cytb6f) and Photosystem II (PSII)complexes are susceptible to SMA solubilisation, suggesting they also reside in lipid-rich environments. Thus,lipid-rich membrane regions could be a general requirement for cytbc1/cytb6fcomplexes, providing a favourablelocal solvent to promote rapid quinol/quinone binding and release at the Q0and Qisites.