Proteomic analysis of the CO₂-concentrating mechanism in the open-ocean cyanobacterium <i>Synechococcus</i> WH8102

Abstract

The open-ocean cyanobacterium Synechococcus WH8102 has recently been sequenced, making high-throughput mass spectrometry(MS)-based proteomics studies possible. Here, we used 1D and 2D gel electrophoresis, matrix-assisted laser desorption–ionization – time-of-flight (MALDI-TOF) MS, and two-hybrid analysis to study the composition and protein interactions within the carboxysome, a protein-rich polyhedral body involved in the CO 2 -concentrating mechanism. We identified the Rubisco large and small subunits and CsoS1 and CsoS2 in the carboxysome-rich particulate fraction and thus conclude the proteins were solely associated with the carboxysome. We also determined that the carboxysome fraction contained numerous membrane-associated proteins, consistent with the presence of membrane contamination. Two-hybrid analysis indicated that CsoS2 and OrfA strongly interacted. They formed dimers and interacted with each other. To our knowledge, this is the first indication of OrfA being biochemically linked to the carboxysome. The ε-class carbonic anhydrase CsoS3 did not interact with other carboxysome components in a binary manner. CsoS3 may not interact, or it may only bind, in fully formed multiprotein complexes. Finally, growth rates and protein expression were unchanged between 100 and 750 µL·L –1 CO 2 .Key words: proteomics, CO 2 -concentrating mechanism, carboxysome, cyanobacteria, Synechococcus.