An immunologic probe for a defined region of the myelin proteolipid

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Identification and characterization of a B-cell determinant within the amphipathic domain (residues 178-238) of the myelin proteolipid protein

Pooled polyclonal rabbit anti-rat myelin and mouse anti-human proteolipid protein (PLP) antisera were screened against a panel of PLP synthetic peptides spanning residues 178-238 of the protein. Cross-reactivity against one determinant defined by PLP(200-219) was particularly prominent in both the anti-myelin and anti-PLP antisera and was chosen for further study. Competitive inhibition studies, utilizing a panel of overlapping synthetic peptides, demonstrated that the C-terminal portion of PLP(200-219), specifically residues comprising PLP(200-217), was important for antibody recognition of this region. Immunohistochemical analyses with an affinity-purified rabbit anti-PLP(200-219) antiserum demonstrated antibody cross-reactivity with PLP in both paraffin- and gelatin-embedded brain sections and immunocytochemical staining of mouse oligodendrocyte-enriched cultures demonstrated antibody binding with native PLP in situ. Staining of living non-permeabilized cells localized binding to the extracellular face of the myelin membrane. Collectively, these data argue for the presence of an immunodominant B-cell determinant defined by PLP residues 200-219. Furthermore, the structural conformation of this determinant in native PLP can be mimicked by the synthetic peptide, resulting in the generation of an antibody reagent that has considerable utility for immunohistochemical and immunocytochemical investigations of PLP expression and localization within the central nervous system myelin membrane.

Immunochemical differentiation of highly conserved forms of myelin proteolipid

1. Myelin proteolipid proteins (PLPs) from five animal species were compared immunochemically. In direct electroblot, ELISA or immunodot analyses, anti-bovine PLP antiserum reacted similarly with bovine, guinea-pig, human, mouse and rat myelin PLP. In contrast, antiserum to bovine BPS4, a fragment obtained by chemical cleavage of bovine PLP, reacted strongly only with the bovine protein. 2. In comparable competitive assays in which antisera were preincubated with PLP from different species prior to reaction with bovine PLP, only preincubation with bovine PLP removed the antibody effectively; PLP from other species competed poorly. 3. The deduced primary structure of human, mouse and rat PLP from their cDNA indicates sequence identity. However, these sequences differ from bovine PLP at four residues, two of them within the BPS4 region. The results suggest that, while anti-PLP antiserum fails to differentiate heterologous from homologous antigen, anti-fragment antiserum can be useful for determining structural relatedness of proteins, especially when used in competitive assays.