An electrophoretic analysis of proteolipids from different rat brain subcellular fractions

Myelin proteolipid protein is not esterified at the site of synthesis

Brain slices prepared from 20-day old rats were incubated with [(3)H]palmitic acid to study its incorporation into myelin proteins. After separation by SDS-PAGE, most of the label was found to be associated with the major proteolipid protein (PLP) and with the intermediate protein (I). The radioactivity measured in PLP at short incubation times was shown to be due to palmitic acid bound to the protein by ester linkages. Time-course incorporation of [(3)H]palmitic acid into PLP of fraction SN(4) (a myelin like membrane) and of purified myelin showed that the former was poorly labeled and no relationship of the type 'precursor-product' between these fractions could be detected. Incorporation of the fatty acid into PLP was not affected by inhibition of the synthesis or transport of myelin PLP with cycloheximide or colchicine, indicating that the pool of PLP that can be acylated must be larger than the extramyelin pool. Addition of unlabeled palmitic acid to the incubation medium, 30 min after the addition of [(3)H]palmitate, stopped the appearance of label in myelin PLP almost immediately, indicating that there is no significant extramyelin pool of PLP destined for transport into myelin. The results presented in this paper strongly suggest that esterification of PLP takes place in the myelin membrane or at a site very close to it.

Mass-spectrometric analysis of myelin proteolipids reveals new features of this family of palmitoylated membrane proteins

In this study, we have investigated the structure of the native myelin proteolipid protein (PLP), DM-20 protein and several low molecular mass proteolipids by mass spectrometry. The various proteolipid species were isolated from bovine spinal cord by size-exclusion and ion-exchange chromatography in organic solvents. Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) of PLP and DM-20 revealed molecular masses of 31.6 and 27.2 kDa, respectively, which is consistent with the presence of six and four molecules of thioester-bound fatty acids. Electrospray ionization-MS analysis of the deacylated proteins in organic solvents produced the predicted molecular masses of the apoproteins (29.9 and 26.1 kDa), demonstrating that palmitoylation is the major post-translational modification of PLP, and that the majority of PLP and DM-20 molecules in the CNS are fully acylated. A series of myelin-associated, palmitoylated proteolipids with molecular masses raging between 12 kDa and 18 kDa were also isolated and subjected to amino acid analysis, fatty acid analysis, N- and C-terminal sequencing, tryptic digestion and peptide mapping by MALDI-TOF-MS. The results clearly showed that these polypeptides correspond to the N-terminal region (residues 1-105/112) and C-terminal region (residues 113/131-276) of the major PLP, and they appear to be produced by natural proteolytic cleavage within the 60 amino acid-long cytoplasmic domain. These proteolipids are not postmortem artifacts of PLP and DM-20, and are differentially distributed across the CNS.

Identification of a new exon in the myelin proteolipid protein gene encoding novel protein isoforms that are restricted to the somata of oligodendrocytes and neurons

The myelin proteolipid protein (PLP) gene (i.e., the PLP/DM20 gene) has been of some interest because of its role in certain human demyelinating diseases, such as Pelizaeus-Merzbacher disease. A substantial amount of evidence, including neuronal pathology in knock-out and transgenic animals, suggests the gene also has functions unrelated to myelin structure, but the products of the gene responsible for these putative functions have not yet been identified. Here we report the identification of a new exon of the PLP/DM20 gene and at least two new products of the gene that contain this exon. The new exon, located between exons 1 and 2, is spliced into PLP and DM20 mRNAs creating a new translation initiation site that generates PLP and DM20 proteins with a 12 amino acid leader sequence. This leader sequence appears to target these proteins to a different cellular compartment within the cell bodies of oligodendrocytes and away from the myelin membranes. Furthermore, these new products are also expressed in a number of neuronal populations within the postnatal mouse brain, including the cerebellum, hippocampus, and olfactory system. We term these products somal-restricted PLP and DM20 proteins to distinguish them from the classic PLP and DM20 proteolipids. They represent putative candidates for some of the nonmyelin-related functions of the PLP/DM20 gene.

Dispersal of proteolipid macroaggregates with trifluoroacetic acid and analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis

The propensity of highly purified proteolipids to form macroaggregates in aqueous solutions, especially when heated with sodium dodecyl sulfate (SDS), with or without thiol reagents, has made qualitative and quantitative analyses of individual species by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) difficult and unreliable. Comparisons of proteolipid profiles from liver, brain, and cultured human keratinocytes demonstrate that 40-72% of the total proteolipid in SDS-PAGE sample buffer is in the form of macroaggregates. Treatment of proteolipids with neat trifluoroacetic acid (TFA) followed by removal of the TFA and incubation in cold SDS-PAGE sample buffer causes complete dispersal of the macroaggregates and allows recovery of virtually all of the proteolipid applied to gels (increasing yields by as much as 3.6 times, depending on tissue type). Gels of TFA-treated samples display differences not only in the relative amounts of individual species but also in novel species not found in untreated samples. Eluted macroaggregates treated with TFA display the same SDS-PAGE banding profiles as TFA-treated whole proteolipids. Hence, routine TFA treatment of proteolipids prior to SDS-PAGE increases total proteolipid yields, allows reliable quantitation of individual apoprotein species, and reveals species previously obscured by the formation of macroaggregates.

Phospholipid composition of myelin and synaptosomal proteolipid from vertebrate brain

1. The phospholipid composition of the main proteolipid complexes of the nervous system was studied in myelin and synaptosomal membranes from brains of representatives of various vertebrate classes. 2. The relative content of acid phospholipids was much higher in proteolipid complexes from myelin and synaptosomal membranes of all vertebrates studied as compared to their content in the initial lipid extract (28-80% and 11-20% of total phospholipid content, respectively). 3. The relative content of acid phospholipids in proteolipid complexes of myelin membranes was much lower in brain of fishes and amphibia as compared to higher vertebrates. 4. The main acid phospholipids of proteolipid complexes was phosphatidylserine, phosphatidic acid being characteristic for myelin proteolipids and diphosphatidyl glycerol for synaptosomal proteolipids of all vertebrates studied.

Synthesis and acylation of myelin proteolipid protein in quaking mouse brain

The synthesis and acylation of proteolipid proteins where investigated in tissue slices prepared from 19-day-old quaking and normal littermate mouse brain. The mutant CNS had a normal rate of total protein synthesis but synthesis of the myelin-specific proteins, proteolipid protein (PLP) and intermediate protein (IP), was impaired to approximately 50% of control. The acylation of myelin proteins with labeled palmitate was reduced to only about 20% of control. The acylation of two nonmyelin proteolipid proteins was also significantly reduced. The incorporation of palmitate into phospholipids was identical in control and mutant. The results indicate impaired synthesis of PLP and IP and a general deficit in protein acylation in quaking brain.

Fatty acid acylation of rat brain myelin proteolipid protein in vitro: identification of the lipid donor

The immediate acyl chain donor for fatty acid esterification of proteolipid protein (PLP) was identified in an in vitro system. Rat brain total membranes, after removal of crude nuclear and mitochondrial fractions, were incubated with radioactive acyl donors, extracted with chloroform/methanol, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of [3H]palmitic acid, CoA, ATP, and Mg2+, acylation of endogenous PLP occurred at a linear rate for at least 2 h. The radioactivity was associated with the protein via an ester linkage, mainly as palmitic acid. Omission of ATP, CoA, Mg2+, or all three reduced fatty acid incorporation into PLP to 44, 27, 8, and 4%, respectively, of the values in the complete system. Incubation of the membrane fraction with [3H]palmitoyl-CoA in the absence of CoA and ATP led to highly labeled PLP. These data demonstrate that activation of free fatty acid is required for acylation. Phospholipids and glycolipids were not able to acylate the PLP directly. Finally, when isolated myelin was incubated with [3H]palmitoyl-CoA in the absence of cofactors, only PLP was labeled, thus confirming the identity of palmitoyl-CoA as the direct acyl chain donor and suggesting that the acylating activity and the PLP pool available for acylation are both in the myelin.

Electrophoretic analysis of myelin proteolipid protein and its deacylated form

Myelin proteolipid protein is known to contain covalently bound fatty acid. To determine the contribution of the fatty acid to the multiple bands observed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the electrophoretic parameters of the proteolipid protein were compared with those of the deacylated form. The relative mobility and proportion of each band, as well as the retardation coefficient and free electrophoretic mobility, were not altered by removal of the fatty acid moiety. Furthermore, the acylated and deacylated forms bound the same amounts of sodium dodecyl sulfate. These data demonstrate that the presence of covalently bound fatty acids does not account for the electrophoretic heterogeneity of the proteolipid.

In vitro synthesis of myelin proteolipid protein from rat brain using a total homogenate system

In vitro synthesis of myelin proteolipid protein (PLP) was explored at different ages using rat brain total homogenates, incubated for 30 min with [3H]glycine. Total proteolipids, extracted from the incubated samples, were separated by SDS-PAGE and the radioactivity was measured in the band corresponding to myelin PLP. The incorporation into PLP in relation to the incorporation into brain total proteins increased from 0.04% at 10 days of age to 0.63% at 20 days, and declined slowly thereafter. Time course experiments were carried out using brain homogenates obtained from rats of 20 days of age (i.e. at the period of maximal synthesis of PLP). Labeled PLP molecules were already found at 2.5 min of incubation and the incorporation of the label into this protein, relative to the incorporation into total proteins, did not vary throughout the entire incubation time (30 min). Pulse-chase experiments using a similar system and adding cycloheximide at different incubation times showed that the appearance of label into mature PLP was immediately blocked by the inhibitor of protein synthesis. These data suggest that PLP is synthesized as such and not as a pre-protein which is subsequently processed to render mature PLP.