Role of Lipids in the Dynamics of Thylakoid Membranes

Abstract

Thylakoid membranes are dynamic systems in which the lateral mobility of proteins and lipids plays a key role in physiological processes including electron transport, regulation of light-harvesting, membrane biogenesis and turnover and repair of proteins. This chapter gives an overview of the importance of lateral mobility in thylakoid membranes, followed by a description of the various methods that can be used to measure diffusion of proteins and lipids in the thylakoid membranes of green plants and cyanobacteria. These methods include Fluorescence Recovery after Photobleaching (FRAP) and Single-Particle Tracking (SPT). We discuss the advantages and limitations of the various methods. We then summarize the information available on lateral diffusion coefficients of proteins and lipids in the thyla-koid membranes of plants and cyanobacteria. Finally we discuss two key ways in which the lipid content of thylakoid membranes influences the lateral mobility of membrane components. Lipid composition, in particular the desaturation of the fatty acyl tails, has a strong influence on membrane fluidity and the phase transition temperature of the membrane. It is also becoming clear that the lipid:protein ratio has a strong influence on membrane properties. Recent work in which isolated granal membranes were “diluted” by fusion with liposomes shows that as the lipid:protein ratio increases, there is a pronounced increase in the mobility of protein complexes in the membrane. However, excessive lipid dilution destabilizes protein supercomplexes, reducing the efficiency of light-harvesting. Clearly there is an optimum density of packing of protein complexes into the membrane, which allows some fluidity in the membrane combined with a high density of photosynthetic complexes and efficient interaction of reaction centers and light-harvesting complexes.