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Physicochemical and Functional Characterization of the C1r Subunit of the First Complement Component¹

From the Department of Molecular Immunology, Scripps Clinic and Research Foundation, La Jolla, California 92037

Abstract

C1r was isolated from serum by an improved method and found to be a glycoprotein with a sedimentation coefficient of 7.0S. Under conditions of physiologic ionic strength and pH, C1r consists of two apparently identical noncovalently linked 95,000 dalton polypeptide chains.

Antisera to C1r detected a protein of -mobility on electrophoresis of serum in agarose in the presence of calcium, and a -mobility protein when the electrophoretic separation was carried out in EDTA. On sucrose gradient ultracentrifugation of normal human serum in the presence of calcium, C1r antigenicity was found in the 19 S region of the gradient. On the other hand, when the gradient contained EDTA, C1r antigenicity was found in the 7 S region. No reaction of anti-C1r with C1r-deficient sera was observed. C1r had a high affinity for active
or proenzyme C1s in the presence of calcium and was able to activate C1s and to form C1 in conjunction with C1q and C1s. Activation of C1s by C1r was inhibited by calcium, C1 inactivator, polyanethol sulfonate, and DFP. Activation of C1s by C1r occurred only after a preliminary incubation of C1r for a brief time at 37°C before addition of C1s. The ability of C1r to form C1 in conjunction with C1q and C1s was, however, progressively lost on incubation at 37°C. Trypsin, although potentiating the activity of crude C1r, did not modify the activity of purified C1r. Its action was on a trypsin-sensitive inhibitor separated from C1r in the final step of the isolation procedure.

The binding of ¹²⁵I-C1r to sensitized sheep erythrocytes required the presence of C1q and calcium but not C1s, whereas the binding of ¹²⁵I-C1s required C1q, C1r and calcium. Thus, C1r functions as not only the activator of C1s, but also serves as the physical link between C1q and C1s in macromolecular C1.

Footnotes

¹ This is Publication No. 1023. This work was supported by United States Public Health Service Grants Al-07007 and CA-14692.

² Recipient of United States Public Health Service Fellowship No. 1 F22 Al 00325.

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