Intracellular localization of a carbonic anhydrase in Porphyridium purpureum (Porphyridiales, Rhodophyta)

The intracellular localization of a carbonic anhydrase (CA) in the red alga , Porphyridium purpureum, was analysed by immunogold labelling experiments. A polyclonal antibody was raised against recombinant CA derived from clone d cDNA. Much higher CA activity was present in cells grown in ordinary air (low-CO2 cells) than in those grown in 5% CO2-enriched air (high-CO2 cells) . An immunoblot analysis detected a single band in the low-CO2 cells, where as no positive band was apparent in the high-CO2 cells. The antibody was us ed as a probe for immunogold labelling. The gold particles were predominant ly localized along the plasma membrane of the low-CO2 cells, whereas no sig nificant labelling was apparent in the high-CO2 cells. The exclusively intr acellular localization of this CA activity and the absence of any membrane- spanning property of the enzyme suggests that the CA was associated with th e cytoplasmic face of the plasma membrane. The results are discussed in rel ation to the possible role of CA.