N-OCT-5 IS GENERATED BY IN-VITRO PROTEOLYSIS OF THE NEURAL POU-DOMAINPROTEIN N-OCT-3

The neural POU-domain proteins N-Oct 3 and N-Oct 5 were first identifi ed in electrophoretic mobility retardation assays through their abilit y to bind to the octamer sequence ATGCAAAT. These two N-Oct factors ar e detected in extracts from tumor-derived and normal neural cells. The y are present differentially, however, in extracts from melanocytes an d melanoma cells: N-Oct 3 is present in extracts from both melanocytes and melanoma cells, whereas N-Oct 5 is more evident in extracts from metastatic melanoma cells. We show here that a cDNA encoding N-Oct 3 d irects synthesis of both the N-Oct 3 and N-Oct 5 proteins and that the N-Oct 5 protein in neural and melanoma-cell extracts is also related to N-Oct 3. N-Oct 5, however, is apparently not expressed in vivo: It is not detected if cells are rapidly lysed in SDS or if extracts are p repared with a cocktail of protease inhibitors that includes the serin e-protease inhibitor 4-(2-Aminoethyl)benzenesulfonyl fluoride hydrochl oride (AEBSF). These data suggest that N-Oct 5 is a specific in vitro proteolytic cleavage product of N-Oct 3 and is not directly related to melanocyte malignancy.