The lateral diffusion of embedded proteins along lipid membranes in protein-poor conditions has been successfully described in terms of the Saffman-Delbrück (SD) model, which predicts that the protein diffusion coefficient D is weakly dependent on its radius R as D ∝ ln(1/R). However, instead of being protein-poor, native cell membranes are extremely crowded with proteins. On the basis of extensive molecular simulations, we here demonstrate that protein crowding of the membrane at physiological levels leads to deviations from the SD relation and to the emergence of a stronger Stokes-like dependence D ∝ 1/R. We propose that this 1/R law mainly arises due to geometrical factors: smaller proteins are able to avoid confinement effects much better than their larger counterparts. The results highlight that the lateral dynamics in the crowded setting found in native membranes is radically different from protein-poor conditions and plays a significant role in formation of functional multiprotein complexes.

Department of Physics University of Helsinki 00014 Helsinki Finland

Institute for Physics and Astronomy University of Potsdam 14476 Potsdam Golm Germany

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences 166 10 Prague Czech Republic

Laboratory of Physics Tampere University of Technology 33101 Tampere Finland

MEMPHYS Centre for Biomembrane Physics University of Southern Denmark 5230 Odense Denmark

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