Co-localization of the myelin-associated glycoprotein and the microfilament components, F-actin and spectrin, in Schwann cells of myelinated nerve fibres.

The myelin-associated glycoprotein (MAG) is an intrinsic membrane protein that is specific for myelinating cells. MAG has been proposed to function in the PNS as an adhesion molecule involved in Schwann cell-axon contact and maintenance of cytoplasmic channels within the myelin sheath. In this report we show that the microfilament components, F-actin and spectrin, co-localize with MAG in periaxonal membranes, Schmidt-Lanterman incisures, paranodal myelin loops, and inner and outer mesaxons of myelinating Schwann cells. F-actin was localized light microscopically by rhodamine-labelled phallicidin binding. Spectrin and MAG were localized by light microscopic and ultrastructural immunocytochemistry. The findings indicate that plasma membrane linkage of F-actin in Schwann cells is likely to occur via spectrin, and raise the possibility that microfilaments interact with the cytoplasmic domain of MAG. An interaction between MAG and microfilaments would be consistent with the proposed function of MAG as an adhesion molecule.