Abstract
The presence of a subset of fast-transported proteins containing sulfate while lacking carbohydrate residues [Stone et al. (1983). J. Neurochem. 41:1085-1089] was confirmed by two-dimensional gel electrophoretic analysis of individual fast-transported proteins double-labeled with 35SO4 and [3H]mannose. Analysis by high-pressure liquid chromatography revealed that the sulfate moieties of these "sulfoproteins" are linked to tyrosine residues. Separation of fast-transported 35SO4-labeled proteins delivered to local regions of axon from proteins en route toward terminal regions demonstrated, on the basis of acid lability of tyrosine-bound sulfate, that the sulfoproteins were localized preferentially in the wavefront of fast-transported proteins. Analysis of individual sulfoproteins confirmed differential transport in that sulfoproteins were present at threefold greater amount in the wavefront than in material off-loaded to local regions of the axon. By contrast, nonsulfated species of molecular weights similar to those of the sulfoproteins were detected in nearly equal amounts in both regions of the transport profile. Treatment of nerve segments containing total 35SO4-labeled fast-transported proteins with sodium carbonate led to solubilization of half the protein-bound sulfate. Exposure of the solubilized proteins to mild acid resulted in the release of approximately 80% of the 35SO4 associated with this fraction. Two-dimensional gel patterns displaying carbonate releasable or nonreleasable fractions are consistent with the most abundantly labeled sulfoproteins being transported within membrane-bound organelles. In terms of apparent destination and subcellular compartmentalization, the sulfoproteins meet critical requirements for consideration as secretable fast-transported proteins.