Identification of essential residues of CTLA-2alpha for inhibitory potency

Purification and characterization of two cysteine proteinase inhibitors from silkworm, Bombyx mori

Cysteine proteinase inhibitors from silkworm are selective inhibitors with low molecular weight and regulate cathepsin L-like cysteine proteinase activity, thus, affecting silkworm metamorphosis. In a previous study, two cysteine proteinase inhibitors, BCPI and BmCPI, were identified in the silkworm genome. To characterize these inhibitors, we expressed and purified them in an Escherichia coli system and analyzed their structure and inhibitory activity in vitro. Both inhibitors showed strong tolerance to high temperature. Their CD spectra revealed that their secondary structures could be recovered by a gradual decrease in temperature. Compared to BCPI, BmCPI exhibited weak inhibitory activity toward cathepsin L. BCPI activity was significantly decreased when its C-terminus was truncated, whereas BmCPI activity increased considerably when the C-terminus tail of BCPI was attached to BmCPI. Additionally, the inhibitory activity of BCPI was strongly reduced if R31 was mutated to A31. In summary, two cysteine proteinase inhibitors from silkworm were characterized in the present study, which facilitates an understanding of the interaction mechanism between cysteine proteinase and its inhibitors in the silkworm.

Cathepsin L coexists with Cytotoxic T-lymphocyte Antigen-2 alpha in distinct regions of the mouse brain

Cathepsins B and L are two prominent members of cystein proteases with broad substrate specificity and are known to be involved in the process of intra- and extra-cellular protein degradation and turnover. The propeptide region of cathepsin L is identical to Cytotoxic T-lymphocyte antigen-2α (CTLA-2α) discovered in mouse activated T-cells and mast cells. CTLA-2α exhibits selective inhibitory activities against papain and cathepsin L. We previously demonstrated the distribution pattern of the CTLA-2α protein in mouse brain by immunohistochemistry, describing that it is preferentially localized within nerve fibre bundles than neuronal cell bodies. In the present study we report colocalization of cathepsin L and CTLA-2α by double labeling immunofluorescence analysis in the mouse brain. In the telencephalon, immunoreactivity was identified in cerebral cortex and subcortical structures, hippocampus and amygdala. Within the diencephalon intense colocalization was detected in stria medullaris of thalamus, mammillothalamic tract, medial habenular nucleus and choroid plexus. Colocalization signals in the mesencephalon were strong in the hypothalamus within supramammillary nucleus and lateroanterior hypothalamic nucleus while in the cerebellum was in the deep white matter, granule cell layer and Purkinje neurons but moderately in stellate, and basket cells of cerebellar cortex. The distribution pattern indicates that the fine equilibrium between synthesis and secretion of cathespin L and CTLA-2α is part of the brain processes to maintain normal growth and development. The functional implication of cathespin L coexistence with CTLA-2α in relation to learning, memory and disease mechanisms is discussed.

Identification and characterization of the interactive proteins with cytotoxic T-lymphocyte antigen-2α

Cytotoxic T-lymphocyte antigen-2α (CTLA-2α) is a potent inhibitor of cathepsin L-like cysteine proteases. Recombinant CTLA-2α is known to be a potent, competitive inhibitor of cathepsin L-like cysteine proteases. In this study, cathepsin L, cathepsin C, and tubulointerstitial nephritis antigen-related protein 1 (TINAGL1) were identified as novel interactive proteins of CTLA-2α by the yeast two-hybrid screening system. The direct interactions and co-localization of these proteins with CTLA-2α were confirmed using co-immunoprecipitation and immunofluorescence staining, respectively. The disulfide-bonded CTLA-2α/cathepsin L complex was isolated from mouse tissue. CTLA-2α was found to be specific and consistently expressed on the maternal side of the mouse placenta. Double immunofluorescence analysis showed that CTLA-2α was co-localized with cathepsin L, cathepsin C, and TINAGL1 in placenta. A simple cell-based fluorescence assay revealed that CTLA-2α exhibited inhibitory activity toward cathepsin C in live cells, which indicated that CTLA-2α is a novel endogenous inhibitor of cathepsin C.

Studies of inhibitory mechanisms of propeptide-like cysteine protease inhibitors

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.

Residue-specific annotation of disorder-to-order transition and cathepsin inhibition of a propeptide-like crammer from D. melanogaster

Drosophila melanogaster crammer is a novel cathepsin inhibitor involved in long-term memory formation. A molten globule-to-ordered structure transition is required for cathepsin inhibition. This study reports the use of alanine scanning to probe the critical residues in the two hydrophobic cores and the salt bridges of crammer in the context of disorder-to-order transition and cathepsin inhibition. Alanine substitution of the aromatic residues W9, Y12, F16, Y20, Y32, and W53 within the hydrophobic cores, and charged residues E8, R28, R29, and E67 in the salt bridges considerably decrease the ability of crammer to inhibit Drosophila cathepsin B (CTSB). Far-UV circular dichroism (CD), intrinsic fluorescence, and nuclear magnetic resonance (NMR) spectroscopies show that removing most of the aromatic and charged side-chains substantially reduces thermostability, alters pH-dependent helix formation, and disrupts the molten globule-to-ordered structure transition. Molecular modeling indicates that W53 in the hydrophobic Core 2 is essential for the interaction between crammer and the prosegment binding loop (PBL) of CTSB; the salt bridge between R28 and E67 is critical for the appropriate alignment of the α-helix 4 toward the CTSB active cleft. The results of this study show detailed residue-specific dissection of folding transition and functional contributions of the hydrophobic cores and salt bridges in crammer, which have hitherto not been characterized for cathepsin inhibition by propeptide-like cysteine protease inhibitors. Because of the involvements of cathepsin inhibitors in neurodegenerative diseases, these structural insights can serve as a template for further development of therapeutic inhibitors against human cathepsins.