Analysis of the genomic and derived protein structure of a novel human serum amyloid A gene, SAA4.

We have determined the structure of the novel SAA gene, SAA4. The gene is 6.2 kb in length and comprises three introns and four exons. Introns 2 and 3 are significantly longer than those of the other human SAA genes. We have sequenced the exons and junction fragments and have shown that the sequence is the same as c-SAA[1] and does not correspond to the pseudogene carried on GSAA4[2]. The predicted SAA4 protein sequence has an eight amino acid insertion relative to the other human SAA proteins and is more closely related to rabbit and mouse SAA proteins than to the other human SAA proteins, or to those of animal species which also possess an insertion. We have analysed the predicted SAA4 protein relative to the other human SAA proteins and have identified three important structural regions. We predict that region 1 of SAA4 represents a lipid binding domain. Region 2 forms an extensive, distinctive, hydrophobic beta sheet region in place of a helical region. In region 3, SAA4 is the only SAA protein having an alpha helix which is not amphipathic. We predict that the SAA4 protein retains a modified function of the conserved region, retains the Ca2+ binding site, has an amino terminal surface site and has a potentially distinct secretion pattern. Together, these differences indicate a distinct function from those of the other SAA proteins.