Karolinska Institute, Department of Medical Nutrition, Huddinge, Sweden.
A study by two-dimensional electrophoresis showed that the soluble, lumenal fraction of Arabidopsis thaliana thylakoids can be resolved into 300 protein spots. After subtraction of low-intensity spots and accounting for low-level stromal contamination, the number of more abundant, lumenal proteins was estimated to be between 30 and 60. Two of these proteins have been identified: a novel plastocyanin that also was the predominant component of the total plastocyanin pool, and a putative ascorbate peroxidase. Import studies showed that these proteins are routed to the thylakoid lumen by the Sec- and delta pH-dependent translocation pathways, respectively. In addition, novel isoforms of PsbO and PsbQ were identified.