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. 2009 May 1;65(Pt 5):482-5.
doi: 10.1107/S174430910901207X. Epub 2009 Apr 24.

Crystallization of Human Complement Component C3b in the Presence of a Staphylococcal Complement-Inhibitor Protein (SCIN)

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Crystallization of Human Complement Component C3b in the Presence of a Staphylococcal Complement-Inhibitor Protein (SCIN)

Brandon L Garcia et al. Acta Crystallogr Sect F Struct Biol Cryst Commun. .
Free PMC article


Staphylococcus aureus secretes a number of small proteins that effectively attenuate the human innate immune response. Among these, the staphylococcal complement-inhibitor protein (SCIN) disrupts the function of the complement component 3 (C3) convertase that is initiated through either the classical or the alternative pathway and thereby prevents amplification of the complement response on the bacterial surface. Recent studies have shown that SCIN may affect the activities of the C3 convertase by binding in an equimolar fashion to C3b, which is itself an integral although non-enzymatic component of the convertase. In order to better understand the nature of the C3b-SCIN interaction, the hanging-drop vapor-diffusion technique was used to crystallize human C3b in the presence of a recombinant form of SCIN. These crystals diffracted synchrotron X-rays to approximately 6 A Bragg spacing and grew in a primitive tetragonal space group (P4(1)2(1)2 or P4(3)2(1)2; unit-cell parameters a = b = 128.03, c = 468.59 A). Cell-content analysis of these crystals was consistent with the presence of either two 1:1 complexes or a single 2:2 assembly in the asymmetric unit, both of which correspond to a solvent content of 51.9%. By making use of these crystals, solution of the C3b-SCIN structure should further our understanding of complement inhibition and immune evasion by this pathogen.


Figure 1
Figure 1
Sample preparation and crystallization. (a) Coomassie-stained SDS–PAGE analysis of the crystallization samples. Electrophoresis was performed under nonreducing conditions. Molecular-mass standards are shown in the left-hand lane, while the crystallization sample is shown in the right-hand lane. The positions of the bands corresponding to C3b and SCIN are indicated. (b) Coomassie-stained native PAGE analysis of C3b alone (lane 1), C3b–SCIN crystallization sample (lane 2) and C3b–C3-9 F(ab) (lane 3) as a positive control for binding. (c) Representative crystals derived from a sample of C3b–SCIN. The scale bar is approximately 50 µm.
Figure 2
Figure 2
Diffraction patterns resulting from the exposure of crystals to synchrotron radiation. The images shown in (a) and (b) are separated by a 45° rotation through the angle ϕ.

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