Ultrastructural Localization of P2 Protein in Actively Myelinating Rat Schwann Cells

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Abstract
The myelin specific protein, P2, was localized immunocytochemically by EM of 4-day-old rat peripheral nerve by a preembedding technique. P2 staining was restricted to Schwann cells that had established a 1-to-1 relationship with an axon. P2 antiserum produced a diffuse staining throughout the entire cytosol of myelinating Schwann cells. The cytoplasmic side of Schwann cell plasma membranes and the membranes of cytoplasmic organelles that were exposed to cytosol were stained by P2 antiserum. This cytoplasmic localization of P2 protein is similar to that for soluble or peripheral membrane proteins that are synthesized on free ribosomes. P2 antiserum stained the cytoplasmic side of Schwann cell membranes that formed single or multiple loose myelin spirals around an axon. In the region of the outer mesaxon, P2 antiserum stained the major dense line of compact myelin. Evidently, P2 protein is located on the cytoplasmic side of compact myelin membranes.