Purification and some properties of rabbit C1r

Area of Science:

• Immunology
• Biochemistry

Background:

• The first component of the complement system (C1) is a large complex crucial for immune responses.
• C1 is composed of subcomponents C1q, C1r, and C1s, with C1r playing a regulatory role.

Purpose of the Study:

• To highly purify the C1r subcomponent from rabbit serum.
• To investigate the role of C1r in the assembly and function of the C1 complex.

Main Methods:

• Affinity chromatography using IgG-Sepharose 6B.
• Column chromatography on CM-Sephadex C-50.
• Analysis of C1 complex reconstitution and binding assays.

Main Results:

• C1r was successfully purified, exhibiting a molecular weight of 105,000 Da, composed of two polypeptide chains (60,000 Da and 45,000 Da) linked by disulfide bonds.
• Purified C1r reconstituted the C1 complex with C1q and C1s in the presence of calcium ions.
• C1r was essential for the binding of C1s to C1q, which is necessary for C1s to bind to sensitized erythrocytes.
• A C1s fragment, lacking part of its H chain, could not bind to C1q even with C1r present, suggesting the H chain's involvement in C1r binding.

Conclusions:

• C1r is indispensable for the formation of the functional C1 complex.
• The binding interaction between C1s and C1r involves a portion of the C1s H chain not critical for enzymatic activity.