Complete primary structures of two major murine serum amyloid A proteins deduced from cDNA sequences

cDNA clones encoding two major mouse serum amyloid A proteins, SAA1 and SAA2, were isolated from a liver cDNA library of the lipopolysaccharide-stimulated BALB/c mouse, and their nucleotide sequences were determined. The insert of the SAA2 cDNA clone contained 607 nucleotides with a 5' untranslated region of 36 nucleotides, a signal peptide region corresponding to 19 amino acids, a mature protein region corresponding to 103 amino acids, and a 3' untranslated region of 202 nucleotides. The SAA1 cDNA insert contained 549 nucleotides specifying a part of a signal peptide region, a mature protein region, and a 3' untranslated region. A comparison of the nucleotide and deduced amino acid sequences of SAA1 cDNA with that of SAA2 cDNA showed a high degree of homology: 95% nucleotide sequence homology in the coding region (91% amino acid sequence homology) and 90% homology in the 3' untranslated region. One of nine amino acid differences between SAA1 and SAA2 predicted from the cDNA sequences was located in a putative proteolytic cleavage site for amyloid A protein formation: SAA2 had the Thr-Met sequence in this site, while SAA1 had the Thr-Ile sequence. This suggests that SAA1, which does not deposit as amyloid A protein, is also potentially susceptible to putative proteolytic enzymes. In addition, as compared with mouse SAA2, human SAA1, monkey and mink amyloid A protein, mouse SAA1 had two unique substitutions, which may play a role in differential deposition of mouse SAA isotypes in amyloid tissues.