INSTABILITY OF THE MAJOR SOLUBLE-ANTIGEN PRODUCED BY RENIBACTERIUM-SALMONINARUM

Using Western blot to examine the nature of soluble antigens produced by Renibacterium salmoninarum, it was found that the major 57-kilodalton (kDa) antigen was unstable. SDS-PAGE of extracellular product (ECP) fractions showed that degradation of the 57-kDa protein increased with time and increased temperature. Several lower molecular mass peptides accumulated temporarily from this degradation. Phenylmethylsulphonyl fluoride prevented breakdown of the 57-kDa protein suggesting a serine protease present in the ECPs was responsible. The results indicated that most, if not all, immunoreactive bands in ECP fractions, other than the 57-kDa protein, arose as a result of degradation of this protein. Western blot analysis of two dimensional gels revealed that the presumptive proteolytic activity was associated with the 57-kDa antigen and several of the apparent degradation products. Many common peptide fragments appeared to be generated from heat-induced proteolysis of these protein moieties, confirming the familial relationship between much of the immunoreactive material in ECP fractions. The results suggested that the 57-kDa antigen is autolytic. Western blot analysis of tissue samples from Atlantic salmon, Salmo salar L., infected with R. salmoninarum suggested that this lability of the 57-kDa antigen also occurred in situ.