Binding of Antibodies onto the Thylakoid Membrane II. Distribution of Lipids and Proteins at the Outer Surface of the Thylakoid Membrane

The number of antibody molecules which stroma-freed chloroplasts can bind out of the mono-specific antisera to monogalactosyl diglyceride, tri-and digalactosyl diglyceride, sulfoquinovosyl diglyceride, phosphatidyl glycerol, sitosterol, plastoquinone, lutein and neoxanthin was determined. This number was compared to the number of antibody molecules which stroma-freed chloroplasts can maximally bind. The result is that the antibodies to the individual lipids cover at most 17 per cent of the accessible thylakoid membrane surface. From a serum which contains both antibodies to the proteins and lipids of the thylakoid membrane, not more antibody molecules are bound than from a serum to the proteins. This means that antibodies to proteins are able to cover up the entire accessible surface of the thylakoids whereas a mixture of antibodies to the lipids, listed above, cover only one forth of the surface. Consequently, antibodies which are bound to proteins can cover up the lipid areas entirely and in turn antibodies which are bound to lipids cover up parts of the protein areas. From this it follows that the portion of the surface, which is made up by lipids must be considerably smaller than 24 per cent. Furthermore, it follows from these experiments that the lipid areas are small and that lipids probably only fill up the gaps between the protein molecules.