A novel human complement-related protein, C1r-like protease (C1r-LP), specifically cleaves pro-C1s

Area of Science:

• Immunology
• Protease biochemistry
• Genomics

Background:

• The human genome sequence enabled the discovery of novel genes.
• The complement system plays a crucial role in innate and adaptive immunity.
• Understanding complement proteases is key to deciphering immune responses.

Purpose of the Study:

• To identify and characterize a novel human complement-related protease, C1r-like protease (C1r-LP).
• To investigate the structure, expression, enzymatic activity, and potential function of C1r-LP.

Main Methods:

• Gene identification using human genome sequence data.
• Protein characterization via cDNA analysis, amino acid identity comparison, and Northern blotting.
• Enzyme activity assays (esterolytic and proteolytic) and inhibition studies.
• Quantification in human serum using ELISA.

Main Results:

• C1r-LP gene identified near the C1r gene, with a significant deletion compared to C1r.
• Predicted C1r-LP protein shows 52% amino acid identity to human C1r and 75% to murine C1r-LP.
• C1r-LP mRNA predominantly expressed in the liver; protein detected in human serum at 5.5 μg/ml.
• Recombinant C1r-LP exhibits esterolytic activity and can cleave pro-C1s, indicating proteolytic function.
• C1r-LP activity is inhibited by di-isopropyl fluorophosphate and C1 inhibitor.

Conclusions:

• C1r-LP is a novel human protease with distinct structural and functional properties.
• C1r-LP possesses proteolytic activity capable of processing pro-C1s, suggesting a role in complement regulation.
• This protease may offer a new mechanism for generating the classical pathway C3/C5 convertase.