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

Abstract

The maximal binding of antibodies out of monospecific antisera to proteins and lipids onto three different chloroplast preparations is compared. In these preparations different parts of the thylakoid membrane surface are accessible to antibodies. Whereas stroma-freed chloroplasts bind antibodies only at the outer surface, also the inner membrane surface is exposed in the two chloroplast preparations which were obtained by ultrasonication and subsequent fractionating centrifugation. In the ultrasonic sediment the inner surface is prevailing. In the ultrasonic supernatant antibodies do not only react with the inner and outer surface but also with con­siderable parties of the interior of the thylakoid membrane. It was found that the thylakoid membrane surface exposed to the stroma consists preponder­antly of proteins whereas the surface directed towards the interior of the thylakoids consists mainly of lipids. All proteins involved in electron transport such as ferredoxin, ferredoxin- NADP+-reductase, plastocyanin, cytochrome f and the coupling factor of photophosphorylation are detectable in the outer surface. The molecules of the coupling factor span the thylakoid membrane from the outside to the inside. They hinder the binding of antibodies to mono- galactosyl diglyceride and to a polypeptide with the apparent molecular weight 24000. The polypeptide 24000 is a major component of the membrane proteins, and is detected on the outer and inner surface of the membrane. The major part of this polypeptide, however, is located in the interior of the thylakoid membrane. The lipid mixture has a different composition in the outer surface than on the inside face of the membrane. The sulfoquinovosyl diglyceride and the phosphatides, phosphatidyl glycerol, phosphatidyl cholin and phosphatidyl inositol are stronger concentrated in the outer surface than in the inside face. The neutral monogalactosyl diglyceride and di- or trigalactosyl diglyceride, however, occur in the inner surface in higher concentrations than in the outer surface. The major part of the sulfoquinovosyl diglyceride and of the monogalactosyl diglyceride are located in the interior of the membrane.