Transthyretin, a new cryptic protease

Area of Science:

• Biochemistry
• Proteomics
• Molecular Biology

Background:

• Transthyretin (TTR) is a plasma protein primarily known for transporting thyroxine (T4) and retinol.
• A portion of TTR circulates bound to apolipoprotein AI (apoA-I) within high-density lipoproteins.

Purpose of the Study:

• To investigate the interaction between TTR and apoA-I.
• To characterize the proteolytic activity of TTR on apoA-I.

Main Methods:

• Proteolytic assays using recombinant and plasma-derived TTR on apoA-I.
• Inhibition studies with serine protease inhibitors.
• Characterization of TTR's catalytic activity using a fluorogenic apoA-I substrate (Abz-ESFKVS-EDDnp).
• Assessment of TTR proteolytic activity in cell lines with and without TTR expression.

Main Results:

• TTR proteolytically cleaves apoA-I at its C-terminus after Phe-225, exhibiting chymotrypsin-like activity.
• TTR's proteolytic activity is inhibited by serine protease inhibitors.
• Optimal conditions for TTR activity were determined (37°C, pH 6.8), with a Km of 29 µM.
• TTR activity is lost when complexed with retinol-binding protein (RBP) but only slightly reduced when complexed with T4.
• TTR-expressing cells demonstrated enhanced degradation of the apoA-I substrate, which was reversed by RBP.

Conclusions:

• Transthyretin (TTR) possesses a novel, cryptic proteolytic activity against apolipoprotein AI.
• This proteolytic function is modulated by TTR's binding partners, RBP and T4.
• TTR may play a previously unrecognized role as a protease in physiological and pathological contexts.