Enzymes that synthesize the IB4 epitope are not sufficient to impart IB4 binding in dorsal root ganglia of rat

The isolectin B₄ (IB4) stains a subset of small and medium-sized dorsal root ganglion (DRG) neurons by binding to terminal α-galactose on glycoproteins and glycolipids. The enzymes α(1,3)galactosyltransferase (1,3GT) and isoglobotriaosylceramide synthase (iGb3S) synthesize the galactose-α(1,3)-galactose group, which is the most common carbohydrate containing terminal α-galactose. 1,3GT preferentially glycosylates proteins whereas iGb3S glycosylates lipids. We generated antibodies against rat 1,3GT and iGb3S that were used for immunohistochemical staining of DRG cells. Virtually all neurons that bound IB4 expressed both enzymes, suggesting that IB4 binds to both glycoproteins and glycolipids in IB4-positive neurons. 1,3GT immunoreactivity was observed in small and medium-sized neurons and satellite cells. iGb3S immunoreactivity was observed in neurons of varying sizes. Many neurons that expressed these enzymes did not bind IB4. Additionally, the majority of neurons that expressed substance P expressed both enzymes but did not bind IB4. Ultrastructual studies revealed that 1,3GT was predominantly associated with the Golgi apparatus, whereas iGb3S was found near the Golgi apparatus and in large, clear vesicles throughout the soma. These data suggest that, although expression of 1,3GT and/or iGb3S appears to be necessary for IB4 binding, expression of these enzymes is not sufficient to impart IB4 binding.