IDENTIFICATION OF THE NEUROTROPHIC FACTOR SEQUENCE OF PROSAPOSIN

Prosaposin, recently identified as a neurotrophic factor (1), is the p recursor of saposins A, B, C, and D. The neurotrophic activity of pros aposin resides in the saposin C domain. We have pinpointed the active sequence to a linear 12-mer located in the NH2-terminal sequence of sa posin C (LIDNNKTEKEIL). Nanomolar concentrations of a 22-mer peptide e ncompassing this region stimulated neurite outgrowth and choline acety ltransferase activity, and prevented cell death in neuroblastoma cells . In primary cerebellar granule cells, the 22-mer also stimulated neur ite outgrowth. Studies of the neuroblastoma line NS20Y using a radiola beled 18-mer from the neurotrophic region identified a high-affinity ( K-d = 70 pM) binding site indicative of receptor-ligand interaction. T he 22-mer stimulated protein phosphorylation of several proteins, some of which were tyrosine-phosphorylated after brief exposure similar to saposin C. Circular dichroism studies demonstrated that the 22-mer wa s converted from a random to a helical structure by addition of gangli oside GM(1). The results are consistent with receptor-ligand binding b y the peptide initiating a signal transduction cascade and resulting i n neuronal differentiation.