Differential phosphorylation of myelin-associated glycoprotein isoforms in cell culture.

The alternative splicing of myelin-associated glycoprotein (MAG) mRNA generates two isoforms that harbor distinct potential phosphorylation sites in their cytoplasmic tails. Here we characterize the in vivo phosphorylation of MAG isoforms in NIH 3T3 cells transfected with the cDNAs encoding the two isoforms of MAG. Our results demonstrate that the longer isoform, L-MAG, is phosphorylated constitutively mainly on serine, but also on threonine and tyrosine residues. This phosphorylation is subject to change by 12-O-tetradecanoylphorbol 13-acetate (TPA) and ammonium vanadate, but not by dibutyryl-cyclic AMP. The shorter isoform, S-MAG, is constitutively phosphorylated only on serine residues. While TPA and dibutyryl-cyclic AMP have no detectable effect, ammonium vanadate induces tyrosine and threonine phosphorylation in S-MAG. 32P labeling of v-src-transformed NIH 3T3 cells that express L-MAG also show that L-MAG is likely to be an in vivo substrate for pp60v-src tyrosine kinase activity. These results demonstrate that both MAG isoforms are phosphorylated in a heterologous cell system and that this phosphorylation is subject to pharmacological manipulation.