Transthyretin, a new cryptic protease

Transthyretin (TTR) is a plasma homotetrameric protein that acts physiologically as a transporter of thyroxine (T-4) and retinol, in the latter case through binding to retinol-binding protein (RBP). A fraction of plasma TTR is carried in high density lipoproteins by binding to apolipoprotein AI (apoA-I). We further investigated the nature of the TTR-apoA-I interaction and found that TTR from different sources (recombinant and plasmatic) is able to process proteolytically apoA-I, cleaving its C terminus after Phe-225. TTR-mediated proteolysis was inhibited by serine protease inhibitors (phenyl-methanesulfonyl fluoride, Pefabloc, diisopropyl fluorophosphate, chymostatin, and N-alpha-p-tosyl-L-phenylalanine-chloromethyl ketone), suggesting a chymotrypsin-like activity. A fluorogenic substrate corresponding to an apoA-I fragment encompassing amino acid residues 223-228 (Abz-ESFKVS-EDDnp) was used to characterize the catalytic activity of TTR, including optimum reaction conditions (37degreesC and pH 6.8) and catalytic constant (K-m=29 muM); when complexed with RBP, TTR activity was lost, whereas when complexed with T-4, only a slight decrease was observed. Cell lines expressing TTR were able to degrade Abz-ESFKVS-EDDnp 2-fold more efficiently than control cells lacking TTR expression; this effect was reversed by the presence of RBP in cell culture media, therefore proving a TTR-specific proteolytic activity. TTR can act as a novel plasma cryptic protease and might have a new, potentially important role under physiological and/or pathological conditions.