PDBsum entry 2qfh

Immune system

PDB id

2qfh

Contents

			Protein chain

					333 a.a.*

* C-alpha coords only

References listed in PDB file

Key reference

Title

The regulatory scr-1/5 and cell surface-Binding scr-16/20 fragments of factor h reveal partially folded-Back solution structures and different self-Associative properties.

Authors

A.I.Okemefuna, H.E.Gilbert, K.M.Griggs, R.J.Ormsby, D.L.Gordon, S.J.Perkins.

Ref.

J Mol Biol, 2008, 375, 80-101. [DOI no: 10.1016/j.jmb.2007.09.026]

PubMed id

18005991

Abstract

Factor H (FH) is a plasma glycoprotein that plays a central role in regulation of the alternative pathway of complement. It is composed of 20 short complement regulator (SCR) domains. The SCR-1/5 fragment is required for decay acceleration and cofactor activity, while the SCR-16/20 fragment possesses binding sites for complement C3d and heparin. X-ray scattering and analytical ultracentrifugation showed that SCR-1/5 was monomeric, while SCR-16/20 formed dimers. The Guinier radius of gyration R(G) of 4.3 nm for SCR-1/5 and those of 4.7 nm and about 7.8 nm for monomeric and dimeric SCR-16/20, respectively, showed that their structures are partially folded back and bent. The distance distribution function P(r) showed that SCR-1/5 has a maximum dimension of 15 nm while monomeric and dimeric SCR-16/20 are 17 nm and about 27 nm long, respectively. The sedimentation coefficient of 2.4 S for SCR-1/5 showed no concentration-dependence, while that for SCR-16/20 was 2.8 S for the monomer and 3.9 S for the dimer. Sedimentation equilibrium data showed that SCR-1/5 is monomeric while SCR-16/20 exhibited a weak monomer-dimer equilibrium with a dissociation constant of 16 microM. The constrained scattering and sedimentation modelling of SCR-1/5 and SCR-16/20 showed that partially folded-back and bent flexible SCR arrangements fitted both data sets better than extended linear arrangements, and that the dimer was best modelled in the SCR-16/20 model by an end-to-end association of two SCR-20 domains. The SCR-1/5 and SCR-16/20 models were conformationally similar to the previously determined partially folded-back structure for intact wild-type FH, hence suggesting a partial explanation of the intact FH structure. Comparison of the SCR-16/20 model with the crystal structure of C3b clarified reasons for the distribution of mutations leading to atypical haemolytic uraemic syndrome.



	Figure 4. Figure 4. Distance distribution function P(r) analyses for SCR-1/5 and SCR-16/20. (a) For SCR-1/5, M represents the most frequent distance within the protein and is 3.5 nm at 0.21 mg/ml, and L represents its maximum dimension at 15 nm. (b) For SCR-16/20 at lower concentrations, M rises from 4.1 nm to 4.8 nm from 0.14 mg/ml to 0.55 mg/ml, and L increases from 17 nm to 19 nm. (c) For primarily dimeric SCR-16/20 at 0.72 mg/ml, 0.86 mg/ml and 1.15 mg/ml, M ranges from 5.1 nm to 5.8 nm and L ranges from 25 nm to 27 nm.		Figure 5. Figure 5. Sedimentation velocity g(s) analyses of SCR-1/5 and SCR-16/20 using DCDT+. (a) The 20 absorbance scans for SCR-1/5 at 0.09 mg/ml and 0.85 mg/ml and a rotor speed of 35,000 r.p.m. gave s^o[20,w] values of 2.50 S and 2.51 S, respectively. The intensity of the g(s) curve at 0.09 mg/ml was increased threefold for clarity. (b) The corresponding 14 and 22 interference scans from the same SCR-1/5 experiment gave s^o[20,w] values of 2.39 S and 2.49 S, respectively. (c) Absorbance data for SCR-16/20 at 0.10 mg/ml (40,000 r.p.m.) and 0.62 mg/ml, 1.33 mg/ml and 1.48 mg/ml (50,000 r.p.m.) showed that the s^o[20,w] values increased from 2.8 S to 3.5 S as the concentration increased. The intensities of the g(s*) curves were equalised for reason of clarity. (d) The corresponding interference data sets for SCR-16/20 showed that the s^o[20,w] values likewise increased from 2.7 S to 3.5 S as the concentration increased.

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