Binding of Antibodies onto the Thylakoid Membrane. V. Distribution of Proteins and Lipids in the Thylakoid Membrane

Binding of antibodies to proteins and lipids onto fragments of the thylakoid membrane was stu­died and compared with the binding of antibodies by stroma-freed chloroplasts. The membrane fragments were prepared from stroma-free chloroplasts by ultrasonication and fractional centrifugation. The fragments have an average diameter of 200 A. Their thickness corresponds to that of the thylakoid membrane. The membrane fragments adsorb out of an antiserum to lipids approximately the same amount of antibodies as out of an antiserum to proteins. In comparison to this, stroma-free chloroplasts bind 4 times more antibodies to proteins than to lipids. From this it follows that the major part of the lipids is located in the membrane surface which is directed towards the inside or is located inside the membrane. As the chemical analysis has shown these results are not caused by an altered chemical composition of the membrane fragments.

Despite the fact that membrane proteins bind considerably less protein antibodies than stroma-free chloroplasts, the antibody binding in membrane fragment might be considerably increased for certain proteins such as a polypeptide with an apparent molecular weight 24000 and cytochrome f. Antibodies to the major components of the lipid mixture, such as to monogalactosyl diglyceride, trigalactosyl diglyceride, sulfoquinovosyl diglyceride and phosphatidyl glycerol are 3 to 4 times more bound by membrane fragments than by stroma-free chloroplasts. From these results it is concluded that the thylakoid membrane surface directed towards the inside is preponderently composed of lipids whereas the surface directed towards the outside consists only by 10 to 15% of lipids.