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The Journal of Immunology, 1976, 117, 630 -634
Copyright © 1976 by The American Association of Immunologists, Inc.
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C3 Requirements for Formation of Alternative Pathway C5 Convertase1

Mohamed R. Daha2, Douglas T. Fearon3 and K. Frank Austen

Departments of Medicine, Harvard Medical School and Robert B. Brigham Hospital, Boston, Massachusetts 02120

Abstract

Although alternative pathway C3 and C5 convertases both have active proteolytic sites dependent on the same protein, Bb, the quantitative requirements for the expression of these activities are sufficiently different to permit their delineation in terms of B input and cell-bound C3b. That the labile component of each active site is Bb was established by their parallel decay rates, regeneration of the original specificities with B in the presence of D, and stabilization of each convertase by C3NeF. The evidence that the spatial relationships of Bb and C3b on the cell surface for C3 and C5 convertase activities are distinct is based not only upon the decay and regeneration of each original convertase but more so upon their interconversion. C3 convertase is converted to C5 convertase by interaction with additional C3 whereas C5 convertase reverts to a C3 convertase by treatment with C3bINA. The capacity of C3bINA treatment to abolish C5 convertase sites without affecting C3 convertase sites indicates the existence of two functional species of C3b, one of which is protected in the C3bBb complex whereas the other is exposed.

Footnotes

1 This work was supported by Grants AI-07722, AI-10356, and AM-05577 from the National Institutes of Health.

2 Postdoctoral Fellow of the Netherlands Organization for the Advancement of Pure Research (ZWO).

3 Postdoctoral Fellow of the Helen Hay Whitney Foundation.




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