Abstract
The membrane polypeptides of growth cone fragments ("growth cone particles," GCPs) isolated from fetal rat brain by subcellular fractionation have been analyzed in further detail. The major polypeptides of salt-washed GCP membranes detected by 1-dimensional gel electrophoresis (Ellis et al., 1985b) resolve in 2-dimensional gels as a spot of 52 kDa that comigrates with beta-tubulin and reacts with anti-beta-tubulin; a 46 kDa, pl 4.3, polypeptide (pp46) that has no equivalent in the soluble fraction and is identical to one of the GCP's major phosphoproteins (Katz et al., 1985) and to GAP43 (Willard et al., 1985); a spot of 42 kDa that comigrates with actin; and a species of 34 kDa (p34) without soluble equivalent. The prominent 38 kDa doublet identified in 1-dimensional gels is difficult to resolve in 2-dimensional gels. The major phosphoproteins pp80ac, pp46, and pp40 (Katz et al., 1985), as well as p34 partition into the oil phase of Triton X-114 extracts, suggesting that they are integral membrane proteins, at least in our experimental conditions. The properties of pp46 reported here are in conflict with the highly hydrophilic amino acid sequence predicted for GAP43/B50/F1 (Basi et al., 1987; Karns et al., 1987). Growth-cone and presynaptic membrane proteins are compared as follows. After eye injection of 35S-methionine, GCPs and synaptosomes are isolated from the target areas of optic nerve of fetal and adult rats, respectively. Polypeptides are separated by 1- and 2-dimensional gel electrophoresis and the radiolabeled species identified fluorographically. The comparison of labeled GCP and synaptosome polypeptides shows that all 5 major Coomassie blue-stained polypeptides of GCP membranes (52, 46, 42, 38, 34 kDa) are intensely labeled after eye injection. However, in synaptosomes, these polypeptides are weakly labeled if at all; instead, an intensely labeled polypeptide of 28 kDa, and several additional species not seen in GCPs, have appeared. Therefore, the major growth cone membrane proteins are developmentally regulated, and the rates of synthesis and transport into the axonal ending of neuronal polypeptides change dramatically at the time of synaptogenesis.