New tandem-repeating peptide structures in polysialoglycoproteins from the unfertilized eggs of kokanee salmon

New polysialoglycoproteins, designated PSGP(On), were isolated from the fertilized and unfertilized eggs of the kokanee salmon, Oncorhynchus nerka adonis. The polysialylglycan chains consisting of alpha-2,8-linked O-acetylated poly(N-glycolylneuraminyl) chains have recently been characterized. We have now determined the complete amino acid sequence of the tandem-repeating units of PSGP(On) from the unfertilized eggs of kokanee salmon and found that the following two distinct forms are present in PSGP(On) in almost identical amounts: [formula: see text] and [formula: see text] where * denotes the O-glycosylation site and mean value of m, n = about 20. Upon fertilization these high-molecular-weight forms of PSGP(On) were proteolytically cleaved to the corresponding repeating units, low-molecular-weight PSGP(On), by the action of a specific protease (PSGPase) at the position two residues set C-terminally to the Pro residue and N-terminally to the Asp residue, i.e. -Pro-Ser-Xaa-Asp-: [formula: see text] and [formula: see text].