Biosynthesis of the Myelin 2',3'-Cyclic Nucleotide 3'-Phosphodiesterases

We have investigated the site of synthesis of the 2',3'-cyclic nucleotide 3'-phosphodiesterases (CNPs I and II) in rat brain. Rapid kinetics of incorporation of CNPs into oligodendrocyte plasma membrane in the intact brain are consistent with their synthesis on free polysomes. This hypothesis was confirmed by the translation in vitro of RNA isolated from free and bound polysomes, respectively. Unlike myelin basic protein (MBP) mRNAs, CNP mRNAs are not enriched in a myelin-associated pool of RNA. MBPs, but not CNPs, were found to readily associate in vitro with membrane vesicles derived from rough endoplasmic reticulum. The avidity of MBPs in binding to membranes is probably related to the previously observed spatial segregation of MBP mRNAs into actively myelinating cellular processes of the oligodendrocyte. Such a segregation would ensure that newly synthesized MBPs are immediately incorporated into myelin. In contrast, the CNPs probably associate with the cytoplasmic surface of the oligodendrocyte plasma membrane through interaction with a membrane-bound receptor.