Extrinsic photosystem II carbonic anhydrase in maize mesophyll chloroplasts

One form of carbonic anhydrase (CA) has been observed in maize (Zea mays) thylakoids and photosystem II (PSII)-enriched membranes. Here, we show that an antibody produced against a thylakoid lumen-targeted CA found in Chlamydomonas reinhardtii reacts with a single 33-kD polypeptide in maize thylakoids. With immunoblot analysis, we found that this single polypeptide could be identified only in mesophyll thylakoids and derived PSII membranes, but not in bundle sheath thylakoids. Likewise, a CA activity assay confirmed a large amount of activity in mesophyll, but not in bundle sheath membranes. Immunoblot analysis and CA activity assay showed that the maximum CA can be obtained in the supernatant of the PSII-enriched membranes washed with 1 m CaCl2, the same procedure used to remove all extrinsic lumenal proteins from PSII. Because this CA reacts with an antibody to lumen-directed CA in C. reinhardtii, and because it can be removed with 1 m CaCl2 wash, we refer to it tentatively as extrinsic CA. This is to distinguish it from another form of CA activity tightly bound to PSII membranes that remains after CaCl2 wash, which has been described previously. The function of extrinsic CA is not clear. It is unlikely to have the same function as the cytoplasmic CA, which has been proposed to increase the HCO3- concentration for phosphoenolpyruvate carboxylase and the C-4 pathway. We suggest that because the extrinsic CA is associated only with thylakoids doing linear electron flow, it could function to produce the CO2 or HCO3- needed for PSII activity.