Simons M, Krämer E M, Thiele C, Stoffel W, Trotter J
Department of Neurobiology, University of Heidelberg, 69120 Heidelberg, Germany.
J Cell Biol. 2000 Oct 2;151(1):143-54. doi: 10.1083/jcb.151.1.143.
Myelin is a specialized membrane enriched in glycosphingolipids and cholesterol that contains a limited spectrum of proteins. We investigated the assembly of myelin components by oligodendrocytes and analyzed the role of lipid-protein interactions in this process. Proteolipid protein (PLP), the major myelin protein, was recovered from cultured oligodendrocytes from a low-density CHAPS-insoluble membrane fraction (CIMF) enriched in myelin lipids. PLP associated with the CIMF after leaving the endoplasmic reticulum but before exiting the Golgi apparatus, suggesting that myelin lipid and protein components assemble in the Golgi complex. The specific association of PLP with myelin lipids in CIMF was supported by the finding that it was efficiently cross-linked to photoactivable cholesterol, but not to phosphatidylcholine, which is underrepresented in both myelin and CIMF. Furthermore, depletion of cholesterol or inhibition of sphingolipid synthesis in oligodendrocytes abolished the association of PLP with CIMF. Thus, PLP may be recruited to myelin rafts, represented by CIMF, via lipid-protein interactions. In contrast to oligodendrocytes, after transfection in BHK cells, PLP is absent from isolated CIMF, suggesting that PLP requires specific lipids for raft association. In mice deficient in the enzyme ceramide galactosyl transferase, which cannot synthesize the main myelin glycosphingolipids, a large fraction of PLP no longer associates with rafts. Formation of a cholesterol- and galactosylceramide-rich membrane domain (myelin rafts) may be critical for the sorting of PLP and assembly of myelin in oligodendrocytes.
髓磷脂是一种富含糖鞘脂和胆固醇的特殊膜结构，其蛋白质种类有限。我们研究了少突胶质细胞对髓磷脂成分的组装，并分析了脂-蛋白相互作用在此过程中的作用。主要的髓磷脂蛋白蛋白脂质蛋白（PLP）是从低密度的富含髓磷脂脂质的CHAPS不溶性膜组分（CIMF）的培养少突胶质细胞中回收的。PLP在内质网离开后但在离开高尔基体之前与CIMF结合，这表明髓磷脂脂质和蛋白质成分在高尔基体复合体中组装。CIMF中PLP与髓磷脂脂质的特异性结合得到以下发现的支持：它能有效地与光可活化胆固醇交联，但不能与磷脂酰胆碱交联，磷脂酰胆碱在髓磷脂和CIMF中含量都较低。此外，少突胶质细胞中胆固醇的消耗或鞘脂合成的抑制消除了PLP与CIMF的结合。因此，PLP可能通过脂-蛋白相互作用被招募到以CIMF为代表的髓磷脂筏中。与少突胶质细胞不同，在BHK细胞中转染后，分离的CIMF中不存在PLP，这表明PLP需要特定的脂质才能与筏结合。在缺乏神经酰胺半乳糖基转移酶（无法合成主要的髓磷脂糖鞘脂）的小鼠中，很大一部分PLP不再与筏结合。富含胆固醇和半乳糖基神经酰胺的膜结构域（髓磷脂筏）的形成可能对少突胶质细胞中PLP的分选和髓磷脂的组装至关重要。
