Characterization of the gene encoding human TAFI (thrombin-activable fibrinolysis inhibitor; plasma procarboxypeptidase B)

Thrombin-activable fibrinolysis inhibitor (TAFI) is a recently described hu man plasma zymogen that is related to pancreatic carboxypeptidase B, The ac tive form of TAFI (TAFla), which is formed by thrombin cleavage of the zymo gen, likely inhibits fibrinolysis by removal from partially degraded fibrin of the carboxyl-terminal lysine residues which act to stimulate plasminoge n activation. We have isolated and characterized genomic clones which encom pass the entire human TAFI gene from lambda phage and bacterial artificial chromosome genomic libraries. The complete TAFI gene contains 11 exons and spans approximately 48 kb of genomic DNA. The positions of intron/exon boun daries are conserved between the TAFI gene and the rat pancreatic carboxype ptidase Al, A2, and B and the human mast cell carboxypeptidase A genes, ind icating that these carboxypeptidases arose from a common ancestral gene. Ho wever, the intron lengths diverge significantly among all of these genes. T he TAFI promoter lacks a consensus TATA sequence, and transcription is init iated from multiple sites. Transient transfection of reporter plasmids cont aining portions of the TAFI 5'-flanking region into mammalian cells allowed localization of the promoter and identified a similar to 70 bp region cruc ial for liver-specific transcription. Sequence analysis of cDNA clones obta ined from human liver RNA indicated that the TAFI transcript is polyadenyla ted at three different sites. Our findings will facilitate the assessment o f the regulation of TAFI expression by transcriptional and/or posttranscrip tional mechanisms. Furthermore, knowledge of the genomic structure of the T AFI gene will aid in the identification of mutations that may he associated with the tendency to either bleed or thrombose.