The thylakoid lumen of chloroplasts. Isolation and characterization

The chloroplast compartment enclosed by the thylakoid membrane, the "lumen," is poorly characterized. The major aims of this work were to design a procedure for the isolation of the thylakoid lumen which could be generally used to characterize lumenal proteins. The preparation was a stepwise procedure in which thylakoid membranes were isolated from intact chloroplasts. Loosely associated thylakoid surface proteins were removed, and following Yeda press fragmentation the lumenal content was recovered in the supernatant following centrifugation. The purity and yield of lumenal proteins were determined using appropriate marker proteins specific for the different chloroplast compartments. Quantitative immunoblot analyses showed that the recovery of soluble lumenal proteins was 60-65% (as judged by the presence of plastocyanin), whereas contamination with stromal enzymes was less than 1% (ribulose-bisphosphate carboxylase) and negligible for thylakoid integral membrane proteins (D1 protein). Approximately 25 polypeptides were recovered in the lumenal fraction, of which several were identified for the first time. Enzymatic measurements and/or amino-terminal sequencing revealed the presence of proteolytic activities, violaxanthin de-epoxidase, polyphenol oxidase, peroxidase, as well as a novel prolyl cis/trans-isomerase.