[Plant purine nucleotide cyclases]

Cyclic nucleotides (cAMP and cGMP) play an essential role in many important cellular processes in prokaryotic and eukaryotic organisms. They are produced by purine nucleotide cyclases: adenylyl and guanylyl cyclases. They are classified as one of two distinct forms: soluble and bound to membranes. Beside the differences in enzyme localization, the domain structure and regulation of enzymes activity are also diverse. However, all cyclases possess three groups of important residues: substrate specifying residue, metal binding residues and transition state stabilization residues. The natural occurrence of cyclic nucleotides in plants is now established. It was shown that in higher plants cNMPs act as a second messengers in a large number of (patho)physiological responses. However, it is only recently that the first plant enzymes with AC and GC activity of the unique structure have been identified and functionally characterized. In this study a systematic analysis of all the known prokaryotic, fungal and animal cyclases was done and direct evidences for the presence AC and GC in plant cells were shown.