Danielle E. Chandler, James Gumbart, John D. Stack, Christophe Chipot, and
Klaus Schulten.
Membrane curvature induced by aggregates of LH2s and monomeric
LH1s.
Biophysical Journal, 97, 2009.
In press.
CHAN2009
The photosynthetic apparatus of purple bacteria is contained within organelles called chromatophores, which form as extensions of the cytoplasmic membrane. The shape of these chromatophores can be spherical (as in Rb. sphaeroides), lamellar (as in Rps. acidophila and Rs. molischianum), or tubular (as in certain Rb. sphaeroides mutants). Chromatophore shape is thought to be induced and maintained by the integral membrane proteins Light Harvesting Complexes I and II (LH1 and LH2), which pack tightly together in the chromatophore. It has been suggested that the shape of LH2, together with its close packing in the membrane, induces membrane curvature, although it is not known why. The mechanism of LH2-induced curvature is explored via molecular dynamics simulations of multiple LH2 complexes in a membrane patch. LH2s from three species - Rb. sphaeroides, Rps. acidophila, and Rs. molischianum - were simulated in different packing arrangements. In each case, the LH2s pack together and tilt with respect to neighboring LH2s in a way that produces an overall curvature. This curvature appears to be caused by a combination of LH2 shape and electrostatic repulsion between cytoplasmic charged residues, the removal of which severely reduces LH2 curvature.
The interaction between LH2s and LH1 monomers is also explored via molecular dynamics simulations; these simulations suggest that curvature is diminished by the presence of LH1 monomers. The implications of our results for chromatophore shape are discussed.
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