Atypical phosphorylcholine-reactive protein from Atlantic salmon, Salmo salar L

A phosphorylcholine-reactive protein was isolated from serum of Atlantic salmon (Salmo salar L.) by affinity chromatography on a phosphorylcholine-conjugated Sepharose column followed by elution with phosphorylcholine. Based on the method used we describe the isolated protein as salmon phosphorylcholine-reactive protein (salmon PRP). Salmon PRP has calcium-independent binding to phosphorylcholine. The protein exists in a monomeric and dimeric form with molecular weight of approximately 80 and 160 kD, respectively. Separation of the protein preparation on SDS-PAGE under reducing conditions resulted in disappearance of the 80 and 160 kD bands and appearance of a major protein band of approximately 100 kD. The N-terminal amino acid sequences of the non-reduced 80 and 160 kD bands and the reduced 100 kD band were identical. Apart from the dimeric form, the molecular weight of salmon PRP and its appearance on SDS-PAGE is similar to human plasminogen. Comparison of the sequence in a protein database resulted in approximately 50% identity with human and bovine plasminogen. In addition, cross-reactivity between antibodies to human plasminogen and salmon PRP was demonstrated. Thus, salmon PRP appears to be different from other phosphorylcholine-reactive proteins which are mostly reported to be CRP-like proteins with calcium-dependent binding to phosphorylcholine, pentameric ring-structure and sequence homology between species. Whether salmon PRP is a new type of phosphorylcholine-binding protein with an unknown function or a plasminogen-like protein with binding specificity for phosphorylcholine calls for further investigation.