Myelin proteolipid protein expressed in COS-1 cells is targeted to actin-associated surfaces

The compact myelin sheath represents one of the largest expanses of membrane-membrane contact in the body and, in the central nervous system, requires the myelin proteolipid protein (PLP) for assembly. To determine whether the molecular properties of PLP promote membrane adhesion and direct its subcellular localization in the absence of oligodendrocyte-specific targeting mechanisms, PLP was expressed in COS-1 fibroblasts. Immunofluorescence staining indicated that PLP was translated effectively, transited the rough endoplasmic reticulum and Golgi apparatus, was delivered to the cell surface, and was endocytosed. In the plasma membrane, the PLP distribution was patchy and only sporadically coincided with sites of membrane-membrane contact between PLP-expressing cells. PLP was not randomly distributed, however, but correlated closely with microfilament locations in leading edge membranes and microvilli, as demonstrated by phalloidin double labeling. Our results indicate that even in non-myelinating cells, PLP can be concentrated in membranes associated with movement and growth, and suggest possible roles for the actin cytoskeleton in PLP localization. As PLP, DM20, and the DM20-like M6 protein all associate with actin-enriched membranes, this may be a common feature of PLP/DM20 gene family members.