Thylakoid Membrane Proteins of Higher Plant Chloroplasts II. Application of Lactoperoxidase Catalyzed Iodination to Study Species Specific Differences and Membrane Topography

Abstract

Based on the result of methodical experiments described in part I of this series, species-specifity of polypeptide patterns as obtained by two different electrophoretic methods was studied. Similarity between Vicia faba and Hordeum vulgare was found mainly with respect to chlorophylla-protein 1, its apoprotein, and some polypeptides with molecular weights below 12,000 dalton and not yet identified. Most of the other thylakoid membrane polypeptides showed differing electrophoretic behaviour when both electrophoretic systems were compared. Covalent labeling of the membrane components by enzymatic iodination was applied in order to improve comparability of electrophoretic results. It is eoncluded that the specific radioaetivity of 125I-labeled polypeptides can be used as an additional criterion, besides electrophoretic. mobility, to improve the interpretation of polypeptide patterns. The results of lactoperoxidase catalyzed iodination were additionally used to analyse membrane topography. The application of the method is critically discussed. It is postulated that the standard technique in combination with thylakoid membrane pre-treatment prior to iodination, aiming at controlled structural variations of the surface, can provide a better insight into membrane structure. Besides proteins, lipids were also found to be labeled after enzymatic iodination of thylakoids. These results are consistent with the existence of boundary lipids associated with the chlorophyll- protein entities as postulated by Anderson (1975).