Atomic Force Microscopy Visualizes Mobility of Photosynthetic Proteins in Grana Thylakoid Membranes

ABSTRACTThylakoid membranes in chloroplasts contain photosynthetic protein complexes that convert light energy into chemical energy. Photosynthetic protein complexes are considered to undergo structural reorganization to maintain the efficiency of photochemical reactions. A detailed description of the mobility of photosynthetic complexes in real-time is necessary to understand how macromolecular organization of the membrane is altered by environmental fluctuations. Here, we used high-speed atomic force microscopy to visualize and characterize the in situ mobility of individual protein complexes in grana thylakoid membranes isolated from Spinacia oleracea. Our observations reveal that these membranes can harbor complexes with at least two distinctive classes of mobility. A large fraction of grana membranes contained proteins with quasi-static mobility, exhibiting molecular displacements smaller than 10 nm2. In the remaining fraction, the protein mobility is variable with molecular displacements of up to 100 nm2. This visualization at high-spatiotemporal resolution enabled us to estimate an average diffusion coefficient of ∼1 nm2 s-1. Interestingly, both confined and Brownian diffusion models could describe the protein mobility of the second group of membranes. We also provide the first direct evidence of rotational diffusion of photosynthetic complexes. The rotational diffusion of photosynthetic complexes could be an adaptive response to the high protein density in the membrane to guarantee the efficiency of electron transfer reactions. This characterization of the mobility of individual photosynthetic complexes in grana membranes establishes a foundation that could be adapted to study the dynamics of the complexes inside the intact and photosynthetically functional thylakoid membranes to be able to understand its structural responses to diverse environmental fluctuations.STATEMENT OF SIGNIFICANCEWe characterized the dynamics of individual photosynthetic protein complexes in grana thylakoid membranes from Spinacia oleracea by high-speed atomic microscopy (HS-AFM). Direct visualization at high spatiotemporal resolution unveils that the mobility of photosynthetic proteins is heterogeneous but governed by the confinement effect imposed by the high protein density in the thylakoid membrane. The photosynthetic complexes display rotational diffusion, which might be a consequence of the crowded environment in the membrane and a mechanism to sustain an efficient electron transfer chain.