Cloning and characterization of the Arabidopsis cyclic phosphodiesterase which hydrolyzes ADP-ribose 1'',2''-cyclic phosphate and nucleoside 2',3'-cyclic phosphates

In eukaryotic cells, pre-tRNAs spliced by a pathway that produces a 3',5'-phosphodiester, 2'-phosphomonoester linkage contain a 2'-phosphate group adjacent to the tRNA anticodon. This S'-phosphate is transferred to NAD to give adenosine diphosphate (ADP)-ribose 1 '',2 ''-cyclic phosphate (Appr>p), which is subsequently metabolized to ADP-ribose 1 ''-phosphate (Appr-1 '' p). The latter reaction is catalyzed by a cyclic phosphodiesterase (CPDase), previously identified in yeast and wheat. In the work presented here, we describe cloning of the Arabidopsis cDNA encoding the 20-kDa CPDase that hydrolyzes Appr>p to Appr-1 '' p. Properties of the bacterially overexpressed and purified Arabidopsis enzyme are similar to those of wheat CPDase. In addition to their transformation of Appr>p, both enzymes hydrolyze nucleoside 2',3'-cyclic phosphates to nucleoside S'-phosphates. For the Arabidopsis CPDase, the apparent K-m values for Appr>p, A>p, C>p, G>p, and U>p are 1.35, 1.34, 2.38, 16.86, and 17.67 mM, respectively. Southern analysis indicated that CPDase in Arabidopsis is encoded by a single copy gene that is expressed, at different levels, in all Arabidopsis organs that were analyzed. Indirect immunofluorescence, performed with transfected protoplasts, showed that CPDase is localized in the cytoplasm. Based on substrate specificity and products generated, the plant enzyme differs from other known cyclic phosphodiesterases. The Arabidopsis CPDase does not have recognizable structural similarity or motifs in common with proteins deposited in public data bases.