2′,3′-Cyclic nucleotide 3′-phosphodiesterase (CNP) is one of the earliest myelin-related proteins to be specifically expressed in differentiating oligodendrocytes (ODCs) in the central nervous system (CNS) and is implicated in myelin biogenesis. CNP possesses an in vitro enzymatic activity, whose in vivo relevance remains to be defined, because substrates with 2′,3,-cyclic termini have not yet been identified. To characterize CNP function better, we previously determined the structure of the CNP catalytic domain by NMR. Interestingly, the structure is remarkably similar to the plant cyclic nucleotide phosphodiesterase (CPDase) from A. thaliana and the bacterial 2′-5′ RNA ligase from T. thermophilus, which are known to play roles in RNA metabolism. Here we show that CNP is an RNA-binding protein. Furthermore, by using precipitation analyses, we demonstrate that CNP associates with poly(A)+ mRNAs in vivo and suppresses translation in vitro in a dose-dependent manner. With SELEX, we isolated RNA aptamers that can suppress the inhibitory effect of CNP on translation. We also demonstrate that CNP1 can bridge an association between tubulin and RNA. These results suggest that CNP1 may regulate expression of mRNAs in ODCs of the CNS. © 2008 Wiley-Liss, Inc.