The prolamellar body is a parachrystaline membrane body. The membrane itself is a lipid bilayer but is highly convoluted giving rise to a body with a three dimensional cubic symmetry, see fig. The prolamellar body is formed in the etioplast by the plastid lipids and mainly one protein, protochlorophyllide reductase. I am interested in the mechanism behind the formation of this cubic membrane structure.
Traditionally membranes are pictured as flat bilayers of lipids with inserted proteins. However there are many examples where the biological membrane are convoluted and form three dimensional networks.

The prolamellar body network is formed by tetrapodal units. With X-ray diffraction measurements we were able to identify the symmetry of the prolamellar body network (2). It has an Fd3m symmetry which is an asymmetric single diamond structure slightly different from the symmetric cubic structures that could be formed by some membrane lipids and water. The deviation from the symmetric double diamond structure in the prolamellar body is probably due to an asymmetric localisation of its membrane protein protochlorofyllide reductase.

 

Protochlorophyllide reductace (POR) is a peripheral protein without a membrane spanning part (1). Structurally it belongs to the group of Short-chain Alcohol Dehydrogenases and itforms a ternary complex with protochloro-phyllide and NADPH. Based on spectroscopic data it has been suggested that the ternary complex forms a dimer or a complex of higher homology in the prolamellar body. In the light protochlorophyllide is reduced to chlorophyllide and then further esterified to chlorophyll. The chlorophyll formation correlates with a transition of the prolamellar body cubic membrane to flat thylakoidmembranes. The critical factor for the transformation is not agreed on.

 

Together with Patrick Williams King's College London UK, I am working on the factors that are regulating the formation of prolamellar body in etioplasts and the disappearance of the prolamellar body during chlorophyll formation. We are using a combination of different spectro-scopic methods, X-ray diffraction and electron microscopy.

 

Birve, S.J., Selstam, E. and Johansson, L.B.-A. () Secondary structure of NADPH:protochlorophyllide oxidoreductase examined by circular dichroism and prediction methods. Biochem J 317:549-555.

 

Williams, W.P., Selstam, E. and Brain, T. () X-ray diffraction studies of the structural organisation of prolamellar bodies isolated from Zea mays. FEBS Lett 422:252-254.

 

Porankiewicz, J., Selstam, E., Campbell, D. and ÷quist, G. () Membrane lipid compossition and restoration of photosynthesis during low temperature acclimation in Synechococccus sp. strain PCC . Physiol. Plant. 104:405-412.

 

Selstam, E., Schelin, J., Brain, T. and Williams, W.P. . The effects of low pH on the properties of protochlorophyllide oxidoreductase and the organization of prolamellar bodies of maize (Zea mays). - Eur. J. Biochem. 269: .