Studies of Inhibitory Mechanisms of Propeptide-Like Cysteine Protease Inhibitors

Abstract EN

Mouse cytotoxic T-lymphocyte antigen-2α (CTLA-2α), Drosophila CTLA-2-like protein (crammer), and Bombyx cysteine protease inhibitor (BCPI) belong to a novel family of cysteine protease inhibitors (I29).

Their inhibitory mechanisms were studied comparatively.

CTLA-2α contains a cysteine residue (C75), which is essential for its inhibitory potency.

The CTLA-2α monomer was converted to a disulfide-bonded dimer in vitro and in vivo.

The dimer was fully inhibitory, but the monomer, which possessed a free thiol residue, was not.

A disulfide-bonded CTLA-2α/cathepsin L complex was isolated, and a cathepsin L subunit with a molecular weight of 24,000 was identified as the interactive enzyme protein.

Crammer also contains a cysteine residue (C72).

Both dimeric and monomeric forms of crammer were inhibitory.

A crammer mutant with Cys72 to alanine (C72A) was fully inhibitory, while the replacement of Gly73 with alanine (G73A) caused a significant loss in inhibitory potency, which suggests a different inhibition mechanism from CTLA-2α.

BCPI does not contain cysteine residue.

C-terminal region (L77-R80) of BCPI was essential for its inhibitory potency.

CTLA-2α was inhibitory in the acidic pH condition but stabilized cathepsin L under neutral pH conditions.

The different inhibition mechanisms and functional considerations of these inhibitors are discussed.