CP12 - A SMALL NUCLEAR-ENCODED CHLOROPLAST PROTEIN PROVIDES NOVEL INSIGHTS INTO HIGHER-PLANT GAPDH EVOLUTION

Higher-plant chloroplast NAD(P)-glyceraldehyde 3-phosphate dehydrogena se (NAD(P)-GAPDH; EC 1.2.1.13) is composed of two different nuclear-en coded subunits, GAPA and GAPB, forming the highly active heterotetrame ric A(2)B(2) enzyme. The main difference between these two subunits is a C-terminal extension of about 30 amino acid residues of GAPB. We pr esent cDNA clones for a nuclear-encoded chloroplast protein from pea, spinach and tobacco, which we have named CP12. The mature protein cons ists of only 74, 75 and 76 amino acid residues, respectively and conta ins two domains with significant homology to the C-terminal extension of GAPB. Affinity chromatography approaches reveal also a specific int eraction between CP12 and chloroplast GAPDH. Northern blot analysis in dicates that CP12 is, like plastid GAPDH, expressed in green and also in etiolated leaves. Further homology is observed between CP12 and ORF 3, an open reading frame located in the hox gene cluster of Anabaena v ariabilis. This gene cluster encodes the subunits of the bidirectional NADP(+)-dependent [NiFeS] dehydrogenase. We propose therefore a commo n evolutionary origin of CP12 and higher-plant chloroplast GAPDH subun it GAPB from the cyanobacterial ORF3.