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Protein Diffusion and Macromolecular Crowding in Thylakoid Membranes1[W]

Summary: Protein Diffusion and Macromolecular Crowding in
Thylakoid Membranes1[W]
Helmut Kirchhoff*, Silvia Haferkamp, John F. Allen, David B.A. Epstein, and Conrad W. Mullineaux
Institute of Botany, 48149 Munster, Germany (H.K., S.H.); School of Biological and Chemical Sciences,
Queen Mary University of London, London E1 4NS, United Kingdom (J.F.A., C.W.M.); and Mathematics
Institute, University of Warwick, Coventry CV4 7AL, United Kingdom (D.B.A.E.)
The photosynthetic light reactions of green plants are mediated by chlorophyll-binding protein complexes located in the
thylakoid membranes within the chloroplasts. Thylakoid membranes have a complex structure, with lateral segregation of
protein complexes into distinct membrane regions known as the grana and the stroma lamellae. It has long been clear that
some protein complexes can diffuse between the grana and the stroma lamellae, and that this movement is important for
processes including membrane biogenesis, regulation of light harvesting, and turnover and repair of the photosynthetic
complexes. In the grana membranes, diffusion may be problematic because the protein complexes are very densely packed
(approximately 75% area occupation) and semicrystalline protein arrays are often observed. To date, direct measurements of
protein diffusion in green plant thylakoids have been lacking. We have developed a form of fluorescence recovery after
photobleaching that allows direct measurement of the diffusion of chlorophyll-protein complexes in isolated grana membranes
from Spinacia oleracea. We show that about 75% of fluorophores are immobile within our measuring period of a few minutes.
We suggest that this immobility is due to a protein network covering a whole grana disc. However, the remaining fraction is
surprisingly mobile (diffusion coefficient 4.6 6 0.4 3 10211


Source: Allen, John F. - School of Biological and Chemical Sciences, Queen Mary, University of London


Collections: Renewable Energy; Biology and Medicine