Golgi sulfation of the oligosaccharide chain of P_o occurs in the presence of myelin assembly but not in its absence

To decipher the intracellular targeting mechanism by which the major glycoprotein of peripheral nerve myelin, P_o, is delivered to myelin after crush injury, as well as to the lysosome after permanent transection injury of the sciatic nerve - experimental paradigms characterized by the presence and absence of axonal regeneration and subsequent myelin assembly, respectively - the role of sulfation of P_o was investigated. P_o sulfation is shown to occur within the Golgi apparatus as a post-translational modification of the oligosaccharide chain which is dependent on processing beyond the action of mannosidase I. It is associated with myelination as observed during development and after crush injury, but does not occur after transection injury, even in the presence of the mannosidase II inhibitor, swainsonine, or the lysosomotrophic agent, L-methionine methyl ester. Although P_o accumulation can be demonstrated with both agents when other precursors are used (e.g. fucose, mannose, amino acids) and indicates lysosomal targeting and delivery of P_o after the action of GIcNAc transferase I, the absence of P_o sulfation after transection suggests that the lack of this modification may result in a default mechanism for lysosomal targeting after nerve transection. Lysosomal degradation of P_o was evaluated after crush injury by pulse-chase analyses with ³⁵SO₄ and [³H] mannose in the presence and absence of chlorate, an inhibitor of ATP-sulfarylase. Although P_o sulfation of the oligosaccharide chain is a stable modification whose labeling is dramatically inhibited by chlorate, no decrease in mannose-labeled P_o was seen with chlorate even with prolonged chase times. Because of this lack of degradation of mannose-labeled P_o in the presence of chlorate in the crushed nerve, it is concluded that the absence of P_o sulfation does not result in a default mechanism for lysosomal delivery.