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The Journal of Immunology, Vol 149, Issue 8 2592-2599, Copyright © 1992 by American Association of Immunologists
ARTICLES |
RG DiScipio
Department of Immunology IMM 18, Scripps Research Institute, La Jolla, CA 92037.
The human complement regulatory protein, factor H, was examined by high resolution transmission electron microscopy. Results of electron microscopy confirm hydrodynamic analysis and indicate that factor H is a monomer of M(r) approximately 155,000. Factor H is an extended flexible molecule with a contour length of 495 A and a cross-sectional diameter of 34 A. Most images of factor H indicate that its polypeptide chain typically folds back on itself with the result that the average length of a factor H molecule is about half its contour length. Only one end of factor H associates with C3b. When bound to C3b, factor H still shows considerable conformational flexibility. Factor I is a bilobal protein of 130 A in length, and its two globular parts have maximal diameters of 54 and 49 A. The results establish that factor I is a two domain protein where the smaller subunit is a protease and the larger one is involved with binding C3b. Factor I binds C3b with a one- to-one stoichiometry in an ionic strength-dependent fashion. In the absence of sodium chloride an affinity constant of 5.7 x 10(5) M-1 was determined for factor I interaction with C3b. Whereas the Scatchard plot of factor I binding to C3b in the absence of factor H is linear, in the presence of factor H a curvilinear graph is obtained. The strong binding sites on C3b for factor I have an affinity at least 15-fold higher in the presence of factor H than in its absence. The results of both electron microscopy and binding studies were combined to compose a scheme envisioning how factors H and I cooperate for the processing of C3b.
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