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Action of the C3b-Inactivator on Cell-Bound C3b¹

From The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138 and the Department of Medicine, Harvard Medical School, Boston, Massachusetts 02115

Abstract

The action of C3bINA and 1H on cell-bound C3b is described in this paper. The -polypeptide of C3b that binds covalently to cell surfaces is cleaved by the C3bINA and 1H into two fragments: one of 60,000 (C3b-60) and another of 40,000 (C3b-40) daltons. The -chain of C3b is unaffected by the C3bINA and 1H. The three polypeptides, C3b-60, C3b-40, and C3, are held together as a single unit by disulfide bonds. This unit, referred to as C3b', is covalently bound to cell surfaces via the C3b-60 polypeptide. The conversion of C3b to C3b' by C3bINA and 1H abolishes the ability of the C3b-bearing cells to adhere to human erythrocytes as well as the ability to form, on the cell surface, the B, &D, and properdin-dependent amplification C3-convertase. However, the agglutinability of the cells with either anti-C3c or anti-C3d is not affected.

Treatment of the C3b'-bearing cells with trypsin releases fragments of C3b' into solution, leaving a polypeptide of 32,000 daltons covalently linked to the membrane. Since the trypsinized cells are agglutinable by anti-C3d but not by anti-C3c, the 32,000 dalton polypeptide appears to correspond antigenically to C3d.

Footnotes

¹ This work was supported by a grant from The American Cancer Society (IM-164) and from The National Science Foundation Program in Human Cell Biology (PCM 77-10206).

² Present address: Department of Genetics, School of Medicine, Washington University, St. Louis, Missouri 63110.

³ Research Career Development Awardee (AI-02245).

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