Nuclear nonO/p54(nrb) protein is a nonclassical carbonic anhydrase

The growing carbonic anhydrase (CA) gene family includes 11 enzymatically a ctive isozymes in mammals. Each of them has a characteristic cellular and s ubcellular distribution pattern. In this report, we demonstrate for the fir st time a nuclear protein with CA activity. A polypeptide recognized by CA II antibodies was purified from several rat tissues using CA inhibitor affi nity chromatography. This polypeptide of apparent 66 kDa mass was character ized using amino acid sequencing and CA activity measurements. It appeared to be identical to nonO/p54(nrb), a previously cloned and characterized RNA and DNA binding nuclear factor. Recombinant nonO generated in baculovirus bound to the CA inhibitor affinity chromatography matrix and revealed detec table CA activity (25 units/mg). Hansson's histochemical staining of rat ly mph nodes followed by light and electron microscopy showed nuclear CA activ ity in lymphocytes, suggesting that the nuclear nonO protein is catalytical ly active in vivo. These results demonstrate that a previously known transc ription factor is a novel, nonclassical Ck Through its CA activity, the non O may function in the maintenance of pH homeostasis in the nucleus.