The PO glycoprotein of peripheral nerve myelin

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Abstract
The PO glycoprotein, the major protein of peripheral nerve myelin, is a hydrophobic glycoprotein which is isolated in soluble and insoluble forms from rabbit sciatic nerve myelin following extensive defatting and mild acidic extraction. The PO glycoprotein was localized exclusively in peripheral nervous system (PNS) myelin of sciatic nerve and rootlets by the immunofluorescent technique using goat anti-PO serum. A single precipitin band occurred in double diffusion and did not cross-react with the myelin basic protein or P2 protein. CNS myelin from brain and spinal cord was negative by the immunofluorescent procedure. The major glycoprotein bands in PNS myelin, in addition to the PO glycoprotein at 28,000, existed at 23,000 and 19,000, as shown by gel electrophoresis in dodecyl sulfate. These glycoproteins, isolated by gel filtration in 2% dodecyl sulfate, showed identity to the PO glycoprotein in their monosaccharide profile and overlapping tryptic peptides on peptide mapping. The 23,000 and 19,000 glycoproteins apparently were derived from the PO glycoprotein by in situ proteolysis; the 23,000 glycoprotein had the identical amino terminal sequence. The 19,000 glycoproteins, beginning with amino-terminal methionine, was identical with the TPO glycoprotein, which reportedly originates from tryptic hydrolysis of the PO glycoprotein in isolated myelin. A tryptic glycopeptide containing 27 amino acids was isolated from the PO glycoprotein and sequenced. It contained a relatively high proportion of aspartic acid (4 residues) and glutamic acid (2 residues) with a high negative charge. The total carbohydrate of the PO, 23,000 and 19,000 glycoproteins apparently exists as a single nonasaccharide moiety linked through N-acetylglucosamine to Asp-14 of the glycopeptide in a N-glycosidic linkage. The PO glycoprotein presumably is a typical amphipathic membrane protein.