Identification of thiol groups and a disulfide crosslink site in bovine myelin proteolipid protein

Misalignment of PLP/DM20 transmembrane domains determines protein misfolding in Pelizaeus-Merzbacher disease

A large number of genetic diseases have been associated with truncated or misfolded membrane proteins trapped in the endoplasmic reticulum (ER). In the ER, they activate the unfolded protein response, which can trigger cell death. Hence, a better understanding of protein misfolding features might help in developing novel therapies. Here, we have studied the molecular basis of Pelizaeus-Merzbacher disease, a leukodystrophy defined by mutations of the PLP1 gene and ER retention of two encoded tetraspan myelin proteins, PLP and DM20. In mouse oligodendroglial cells, mutant isoforms of PLP/DM20 with fewer than all four transmembrane (TM) domains are fully ER retained. Surprisingly, a truncated PLP with only two N-terminal TM domains shows normal cell-surface expression when coexpressed with a second truncated PLP harboring the two C-terminal TM domains. This striking ability to properly self-align the TM domains is disease relevant, as shown for the smaller splice isoform DM20. Here, the increased length of TM domain 3 allows for compensation of the effect of several PLP1 point mutations that impose a conformational constraint onto the adjacent extracellular loop region. We conclude that an important determinant in the quality control of polytopic membrane proteins is the free alignment of their TM domains.

Myelin proteolipid protein--the first 50 years

Myelin proteolipid protein (PLP), the most abundant protein of central nervous system (CNS) myelin, is a hydrophobic integral membrane protein. Because of its physical properties, which make it difficult to work with, progress towards determining the exact function(s) and disease associations of myelin PLP has been slow. However, recent molecular biology advances have given new life to investigations of PLP, and suggest that it has multiple functions within myelin and is of importance in several neurological disorders.

Redefining the lipophilin family of proteolipid proteins

The past few years have seen a dramatic increase in our understanding, in molecular terms, of the involvement of the central nervous system proteolipid protein in myelinogenesis and X-linked genetic diseases. In addition, we have expanded our knowledge of the proteins that have been recruited into the vertebrate myelin membrane over the past 400 million years with the molecular cloning of several cDNAs encoding proteins which are homologous to the proteolipid protein gene. In searching for a name to distinguish these proteins from other "proteolipid" proteins of nonneural origin I propose that we resurrect the term "lipophilins" which describes a small family of unusually hydrophobic integral membrane proteins exhibiting identical topologies and similar physical properties. Two subgroups are distinguishable among the lipophilins based on the patterns of expression during development and the presence or absence of a small motif that is exposed to the extracellular space.

Monoclonal antibody O10 defines a conformationally sensitive cell-surface epitope of proteolipid protein (PLP): evidence that PLP misfolding underlies dysmyelination in mutant mice

Mutations in the gene for proteolipid protein (PLP) have been associated with CNS dysmyelination and abnormal oligodendrocyte death in spontaneous mouse mutants and in Pelizaeus-Merzbacher disease; however, the effect of mutations on PLP structure and function are little understood. We have identified a monoclonal antibody directed against a novel cell surface epitope of PLP, termed O10. By immunofluorescence analysis, COS-7 cells transiently transfected to express PLP (or its isoform DM20) can be stained with antibody O10 and another antibody (A431) directed against the C terminus of PLP/DM20. The subcellular distribution of immunofluorescence labels for the two antibodies is not identical, suggesting that the O10 epitope is acquired post-translationally. When PLP/DM20 from jimpy, jimpymsd, and rumpshaker mutant mice is expressed in COS-7 cells and compared with wild-type PLP/DM20, none of the mutant isoforms displays the O10 epitope, whereas the C-terminal epitope is detected. Because the O10 but not the A431 epitope is also sensitive to SDS and reducing agents, this strongly suggests abnormal protein folding in the PLP mutants. PLP from jimpymsd mice is obviously misfolded, because the amino acid substitution (Ala242 --> Val) is located within a transmembrane domain to which the O10 antibody does not bind. We propose that the O10 epitope emerges as the full length protein reaches a functional tertiary structure and that the absence of this epitope marks a structural defect of PLP that leads to dysmyelination.

Conservation of topology, but not conformation, of the proteolipid proteins of the myelin sheath

The proteolipid protein gene products DM-20 and PLP are adhesive intrinsic membrane proteins that make up >/=50% of the protein in myelin and serve to stabilize compact myelin sheaths at the extracellular surfaces of apposed membrane lamellae. To identify which domains of DM-20 and PLP are positioned topologically in the extracellular space to participate in adhesion, we engineered N-glycosylation consensus sites into the hydrophilic segments and determined the extent of glycosylation. In addition, we assessed the presence of two translocation stop-transfer signals and, finally, mapped the extracellular and cytoplasmic dispositions of four antibody epitopes. We find that the topologies of DM-20 and PLP are identical, with both proteins possessing four transmembrane domains and N and C termini exposed to the cytoplasm. Consistent with this notion, DM-20 and PLP contain within their N- and C-terminal halves independent stop-transfer signals for insertion into the bilayer of the rough endoplasmic reticulum during de novo synthesis. Surprisingly, the conformation (as opposed to topology) of DM-20 and PLP may differ, which has been inferred from the divergent effects that many missense mutations have on the intracellular trafficking of these two isoforms. The 35 amino acid cytoplasmic peptide in PLP, which distinguishes this protein from DM-20, imparts a sensitivity to mutations in extracellular domains. This peptide may normally function during myelinogenesis to detect conformational changes originating across the bilayer from extracellular PLP interactions in trans and trigger intracellular events such as membrane compaction in the cytoplasmic compartment.

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.

Monoclonal antibodies against myelin proteolipid protein: identification and characterization of two major determinants

This report describes the preparation and characterization of a panel of monoclonal antibodies (mAbs) against the myelin proteolipid protein (PLP). A Lewis rat was immunized with bovine proteolipid apoprotein and 27 mAbs were selected based on their reactivity against bovine PLP on enzyme-linked immunosorbent assays. Eleven mAbs recognized the PLP carboxyl-terminal sequence when tested against a panel of synthetic peptides in a solid-phase assay. A carboxyl-terminal pentapeptide (residues 272-276) was sufficient for antibody binding and the terminal phenylalanine residue was found particularly important. Deletion, modification, or replacement of this residue markedly reduced or obliterated antigen-antibody interaction. Nine mAbs reacted with a second antigenic determinant, residues 209-217, but these could be identified only by competitive immunoassays. This peptide was a more effective inhibitor than the longer peptides 202-217 and 205-221, suggesting that flanking residues may interfere with peptide-antibody interaction. Seven antibodies did not react with any of the synthetic peptides tested and their determinants remain unidentified. Immunoblot analysis showed that the mAbs reacted with both the PLP and the DM-20 isoforms. Twenty-three of the mAbs were of the immunoglobulin G2a or b isotype; the remaining antibodies were immunoglobulin M and all of these were specific for residues 209-217. Cultured murine oligodendrocytes were stained by most of the mAbs tested, but the most intense reactivity was observed with the carboxyl-terminus-specific mAbs. The immunocytochemical analyses demonstrate that the mAbs react with the native PLP in situ and show their potential usefulness for studies of the cell biology of myelin and oligodendrocytes.

Cysteine-108 is an acylation site in myelin proteolipid protein

Myelin proteolipid protein (PLP) contains covalently bound long-chain fatty acids. A large proportion of these acyl moieties are bound in thioester linkages, as demonstrated by alkylation of newly formed SH groups upon deacylation. To identify the Cys residue(s) involved in the thioester linkage(s), reduced and carboxyamidomethylated proteolipid protein was labeled with [14C]iodoacetamide upon deacylation with neutral hydroxylamine. The labeled protein was digested with trypsin or pepsin, and peptides analyzed by RP-HPLC. Identification of the isolated radioactive peptides by amino acid analysis, peptide sequencing and/or fast-atom bombardment-mass spectrometry revealed that Cys108 in the bovine PLP sequence is an acylated site. The sequence surrounding the palmitoylation site in the myelin PLP is strikingly similar to that found in rhodopsin. Furthermore, as in rhodopsin and other members of the G protein-coupled receptor family, this Cys residue is located within a hydrophilic, basic, and possibly cytoplasmic, domain.