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Protein Shape and Crowding Drive Domain Formation and Curvature in Biological Membranes

Raoul N. Frese ^*  , Josep C. Pàmies , John D. Olsen ^¶, Svetlana Bahatyrova , Chantal D. van der Weij-de Wit , Thijs J. Aartsma ^*, Cees Otto , C. Neil Hunter ^¶, Daan Frenkel  and Rienk van Grondelle 

^* Biophysics, Faculty of Mathematics and Natural Sciences, Leiden University, 2300RA Leiden, The Netherlands; Biophysics, Faculty of Sciences, Vrije Universiteit Amsterdam, de Boelelaan 1081, 1081HV, The Netherlands; Biophysical Techniques Group, Department of Science and Technology, University of Twente, 7500AE Enschede, The Netherlands; FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098SJ Amsterdam, The Netherlands; and ^¶ Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom

Correspondence: Address reprint requests to Raoul N. Frese, E-mail: frese{at}physics.leidenuniv.nl.

Folding, curvature, and domain formation are characteristics of many biological membranes. Yet the mechanisms that drive both curvature and the formation of specialized domains enriched in particular protein complexes are unknown. For this reason, studies in membranes whose shape and organization are known under physiological conditions are of great value. We therefore conducted atomic force microscopy and polarized spectroscopy experiments on membranes of the photosynthetic bacterium Rhodobacter sphaeroides. These membranes are densely populated with peripheral light harvesting (LH2) complexes, physically and functionally connected to dimeric reaction center-light harvesting (RC-LH1-PufX) complexes. Here, we show that even when converting the dimeric RC-LH1-PufX complex into RC-LH1 monomers by deleting the gene encoding PufX, both the appearance of protein domains and the associated membrane curvature are retained. This suggests that a general mechanism may govern membrane organization and shape. Monte Carlo simulations of a membrane model accounting for crowding and protein geometry alone confirm that these features are sufficient to induce domain formation and membrane curvature. Our results suggest that coexisting ordered and fluid domains of like proteins can arise solely from asymmetries in protein size and shape, without the need to invoke specific interactions. Functionally, coexisting domains of different fluidity are of enormous importance to allow for diffusive processes to occur in crowded conditions.

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