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A Phorbol Ester‐Sensitive Kinase Catalyzes the Phosphorylation of P 0 Glycoprotein in Myelin
Author(s) -
Brunden Kurt R.,
Poduslo Joseph F.
Publication year - 1987
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1987.tb02448.x
Subject(s) - phosphorylation , endoplasmic reticulum , myelin , biochemistry , biosynthesis , cycloheximide , glycoprotein , golgi apparatus , sciatic nerve , serine , protein phosphorylation , kinase , chemistry , protein biosynthesis , axoplasmic transport , biology , microbiology and biotechnology , protein kinase a , enzyme , endocrinology , central nervous system , anatomy
The proposed structural protein of peripheral nerve myelin, P 0 , has been shown to have several covalent modifications. In addition to being glycosylated, sulfated, and acylated, P 0 is phosphorylated, with the intracellular site of this latter addition being in question. By employing nerve injury models that exhibit different levels of P 0 biosynthesis in the absence and presence of myelin assembly, we have examined the cellular location of P 0 phosphorylation. It is demonstrated that there is comparable P 0 phosphorylation in both normal and crush‐injured adult rat sciatic nerves, although the level of biosynthesis of P 0 differs between these myelin maintaining and actively myelinating nerve models, respectively. The glycoprotein does not appear to be phosphorylated readily in the transected adult sciatic nerve, a preparation in which P 0 biosynthesis is observed but that lacks myelin membrane. These observations suggest that the modification is not associated with the biosynthesis or maturation of P 0 in the endoplasmic reticulum or Golgi, but that it instead occurs after myelin assembly. That P 0 phosphorylation occurs in the normal nerve even when translation is inhibited by cycloheximide treatment lends further support to this conclusion. P 0 is shown to be phosphorylated on one or more serine residues, with all or most of the phosphate group(s) being labile as evidenced by pulse‐chase analysis. Addition of a biologically active phorbol ester, 12‐ O ‐tetradecanoylphorbol‐13‐acetate or 4β‐phorbol 12, 13‐dibutyrate, substantially increases the extent of [ 32 P]orthophosphate incorporation into the glycoprotein of normal and crushed nerve but not transected nerve. Biologically inactive 4α‐phorbol 12, 13‐didecanoate has no effect on P 0 phosphorylation. Similarly, the addition of the cyclic AMP analog 8‐bromo‐cyclic AMP causes no appreciable changes in P 0 labeling. These findings indicate that the phorbol ester‐sensitive enzyme, protein kinase C, may be responsible for the phosphorylation of P 0 within the myelin membrane.