Subcellular localization of the BtpA protein in the cyanobacterium Synechocystis sp. PCC 6803

Photosystem I is a large pigment-protein complex embedded in the thylakoid membranes of chloroplasts and cyanobacteria. In the cyanobacterium Synechoc ystis sp. PCC 6803, the btpA gene encodes a 30-kDa polypeptide. Mutations i n this gene significantly affect accumulation of the reaction center protei ns of photosystem I in Synechocystis 6803 [Bartsevich, V. V. & Pakrasi, H. B. (1997) J. Biol. Chem. 272, 6372-6378]. We describe here the intracellula r localization of the BtpA protein. Immunolocalization in Synechocystis 680 3 cells demonstrated that the BtpA protein is tightly associated with the t hylakoid membranes. Phase fractionation in the detergent Triton X-114 indic ated that BtpA is a peripheral membrane protein. To determine which surface of the thylakoid membrane BtpA is exposed to, we used a two-phase polymer partitioning technique to develop a novel method to isolate inside-out and right-side-out thylakoid vesicles from Synechocystis 6803. Treatments of su ch vesicles with different salts and protease showed that the BtpA protein is an extrinsic membrane protein which is exposed to the cytoplasmic face o f the thylakoid membrane.