2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP or CNPase)This family consists of the eukaryotic protein 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) is one of the earliest myelin-related proteins expressed in differentiating oligodendrocytes and Schwann cells. CNP is abundant in the central nervous system and in oligodendrocytes. This protein is also found in mammalian photoreceptor cells, testis and lymphocytes. Although the biological function of CNP is unknown, it is thought to play a significant role in the formation of the myelin sheath, where it comprises 4% of total protein. CNP selectively cleaves 2',3'-cyclic nucleotides to produce 2'-nucleotides in vitro. Although physiologically relevant substrates with 2',3'-cyclic termini are still unknown, numerous cyclic phosphate containing RNAs occur transiently within eukaryotic cells. Other known protein families capable of hydrolysing 2',3'-cyclic nucleotides include tRNA ligases and plant cyclic phosphodiesterases. The catalytic domains from all these proteins contain two tetra-peptide motifs H-X-T/S-X, where X is usually a hydrophobic residue. Mutation of either histidine in CNP abolishes enzymatic activity. CNPases belong to the 2H phosphoesterase superfamily. They share a common active site, characterized by two conserved histidines, with the bacterial tRNA-ligating enzyme LigT, vertebrate myelin-associated 2',3' phosphodiesterases, plant Arabidopsis thaliana CPDases and several several bacteria and virus proteins.