Effects of cations on the adhesion between membrane vesicles obtained by digitonin fractionation of spinach chloroplasts

The heavy fraction obtained by digitonin treatment of stacked spinach chloroplasts, suspended in media with different ionic composition, was examined by electron microscopy. In the presence of 5 mM MgCl₂ the thylakoid fragments adhere to one another in a ‘stacked configuration,’ while, in the presence of 10 mM NaCl, mainly only single ‘unstacked’ vesicles are present, which, upon addition of 5 mM MgCl₂, completely revert to the stacked configuration. As previously reported (Chow, W.S. and Barber, J. (1980) Biochim. Biophys. Acta 593, 149–157), no difference in fractionation of chlorophyll between light and heavy fractions was seen after a second digitonin treatment of this fraction suspended in media containing different cation concentrations. From these results it was concluded: (1) that for the unstacking process the movement of proteins or complexes from the stromal to the granal lamellae is not required. Upon lowering the screening by cations of the surface negative charges, the membranes separate from one another; (2) that, under these conditions, as in others (Jennings, R.C., Gerola, P.D., Garlaschi, F.M. and Forti, G. (1980) FEBS Lett. 115, 39–42), digitonin fractionation is not a tool to investigate the degree of membrane stacking.