Myelin-associated glycoprotein, MAG, selectively binds several neuronal proteins

Myelin-associated glycoprotein (MAG) is a potent inhibitor of axonal regeneration and also, depending on the age and type of neuron, can promote axonal growth. In addition, MAG influences stability of both myelin and the axon in the intact, mature nervous system. The identity of the neuron/axonal MAG-binding receptor responsible for effecting these responses is not known. Here we show that a soluble, chimeric form of MAG, MAG-Fc, can bind to the neuronal cell body and neurites equally well, in a sialic acid-dependent manner. Importantly, MAG-Fc specifically precipitates a number of surface proteins from post-natal cerebellar, dorsal root ganglion (DRG) and PC12 neurons. These proteins are not precipitated by a control Fc-containing chimera and are not apparent when a MAG antibody is included in the precipitation mix as a competitive inhibitor. Based on molecular weight, two prominent proteins of 190 and 250 kD are precipitated from all three neuron types. The 190 kD protein is a sialoglycoprotein, since it is not apparent in the precipitate from neurons which have been desialylated. Other proteins are precipitated but are less abundant and are different for each type of neuron. One or more of these proteins is/are likely to be the functional MAG receptor.