A unique insertion in the primary structure of bovine amyloid AA protein

Amyloid fibrils were isolated from kidney tissue of a cow afflicted with renal failure caused by spontaneous reactive amyloidosis. These fibrils were reduced and alkylated, and the amyloid subunit protein was isolated on a column of Sepharose CL6B. The protein was fragmented with both trypsin and Staphylococcus protease, and the resultant peptides were separated by high-performance liquid chromatography. Sequence analysis gave the complete primary structure of the protein with overlaps of the tryptic peptides confirmed by the Staphylococcus protease peptides. Comparison of the bovine amyloid A (AA) amino acid sequence with human protein AA demonstrates complete invariability from human position 33 to 45 and a very high degree of homology from positions 16 to 29 and 46 to 63. These data indicate that these portions of the molecule may be significant factors in amyloid fibrilogenesis. The bovine AA protein shows a blocked amino terminus, as is the case with the dog and the cat AA proteins. In addition, this protein contains an insertion of nine amino acid residues between human positions 69 and 70. The existence of an additional six residues after position 76 makes the bovine AA an unusually large 90 amino acid peptide. These findings point to a high tolerance for mutation in the carboxyl end of the molecule.