cDNA cloning and sequencing of rat alpha 1-macroglobulin

cDNA clones coding for the plasma protease inhibitor alpha 1-macroglobulin were isolated from a rat liver library. The obtained cDNA sequence contained 4701 nucleotides and had an open reading frame coding for a 1,500 amino acid long protein, including a 24 amino acid signal peptide. The identity of the deduced protein sequence as alpha 1-macroglobulin was established by comparison with published peptide sequences of the protein. The mature protein shares 53% and 57% overall amino acid identity with the two other identified members of the rat alpha-macroglobulin family, alpha 1-inhibitor 3 and alpha 2-macroglobulin. A sequence typical for an internal thiol ester was identified. Of the 24 cysteines, 23 are conserved with alpha 2-macroglobulin. However, instead of the two most C-terminal cysteines in alpha 2-macroglobulin, which forms a disulfide bridge in the receptor binding domain, alpha 1-macroglobulin contains phenylalanine. One mRNA species hybridizing with the alpha 1-macroglobulin probe was observed in rat and mouse liver RNA (approximately 6.2 kb), whereas no corresponding transcript was detected in RNA from human liver.