Abstract
C[complementary]DNA clones encoding 2 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 9 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. SAA1, which does not deposit as amyloid A protein, is also potentially susceptible to putative proteolytic enzymes. As compared with mouse SAA2, human SAA1, monkey and mink amyloid A protein, mouse SAA1 had 2 unique substitutions, which may play a role in differential deposition of mouse SAA isotypes in amyloid tissues.