Intracellular transport and sorting of the oligodendrocyte transmembrane proteolipid protein.

Delineating the properties and functions of the major central nervous system myelin proteins has been the focus of intensive research for decades. For PLP, this task has been confounded by its unusual properties, the complexity of the cellular membrane in which it resides, and the absence of a functional assay for the protein. The development of new experimental paradigms in which to study PLP may shed fresh light on the properties and functions of this intrinsic membrane protein. In the present communication we have used indirect, double label, immunofluorescence, and confocal microscopy to examine the distribution of PLP in Cos-7 cells transfected with an expression vector bearing the human PLP cDNA. Our results show that PLP is synthesized in the rough endoplasmic reticulum of transfected cells and passes through the Golgi apparatus to the cell surface. These results are consistent with previous studies showing PLP reaches the cell surface by transport through the secretory pathway. Levels of PLP at the cell surface are modest, most likely because protein deposited in this compartment can be endocytosed and subsequently transported to perinuclear lysosomes. Similar results are reported in the companion communication by Sinoway et al. (J Neurosci Res, 37:551-562, 1994). Using transfected HeLa cells they show that DM20 alone and PLP coexpressed with DM20 assume appropriate conformations for transport to the cell surface. The presence of PLP in subcellular compartments beyond the endoplasmic reticulum in Cos-7 cells indicates that the protein achieves a conformation appropriate for transport in the absence of other oligodendrocyte-specific factors; however, accumulation of large amounts of PLP in the cytoplasmic membrane compartment may require interactions with such glial-specific factors. Thus, the transfection paradigm described herein should prove a useful tool for investigating the folding and sorting of wild type and mutant forms of PLP as well as its membrane topology and posttranslational processing.