Expression of a 48-Kilodalton Growth-Associated Protein in the Goldfish Retina

One of the most striking molecular correlates of optic nerve regeneration in the goldfish is the increased labeling of a 48 kilodalton (kD) acidic protein that is conveyed to the developing nerve endings from the retina by rapid axonal transport. The present study examined the biosynthesis and molecular characteristics of this protein. Retinas derived either from intact controls or from goldfish undergoing optic nerve regeneration (10-14 days postcrush) were pulse-labeled with 3H]proline or 35S]methionine, followed by subcellular fractionation and analysis of protein synthesis patterns by two-dimensional gel electrophoresis and fluorography. Synthesis of the 48-kD acidic protein (termed here GAP-48) was detected only in retinas that were undergoing axonal regeneration. Pulse-chase labeling experiments demonstrated that the protein undergoes a post-translational modification that requires 15-20 min. This processing could be selectively blocked by tunicamycin, an inhibitor of protein N-glycosylation. The protein was also found to incorporate low levels of phosphate in vitro. Thus, the differential appearance of GAP-48 in regenerating axons might be regulated either at the level of gene expression or by selective posttranslational processing in retinal ganglion cells. By the criteria of molecular weight, isoelectric point, anomalous migration properties on sodium dodecyl sulfate-polyacrylamide gels, phosphorylation, subcellular distribution, and the pattern of digestion products generated by Staphylococcus aureus V8 protease, GAP-48 appears to be equivalent to the B-50 (F-1) phosphoprotein of the mammalian brain.