Neutron scattering study of the human complement proteins C3, C4 and C5 and the methylamine derivative of C4

Small-angle scattering study of alpha 1 inhibitor III from rat blood plasma

The alpha 1 proteinase inhibitor III from rat blood plasma, homologous to the alpha 2-macroglobulin family of proteins, has been studied in solution using small-angle scattering of X-rays and of neutrons: the radius of gyration, Rg, was found to be 4.5 nm, and the largest distance within the molecule, Dmax = 14 nm. When the inhibitor reacts with chymotrypsin or methylamine, the resulting derivatives yield slightly higher Rg-values, 4.7 and 4.85 nm, respectively. The data of the native protein are consistent with a model, the projection of which resembles the letter V and which is formed by the two identical halves of an elliptic cylinder with semi-axes of 2.1 and 5.5 nm and a length of 11 nm. This elliptic cylinder model also explained the scattering from the monomeric complement proteins C3 and C4, as well as that from the monomers of the dimeric and tetrameric alpha 2-macroglobulin family of proteins (Osterberg, R., et al. (1991), Biochemistry 30, 7873-7878). Due to the conformational change occurring when the thiol ester bond is split, the cleft in the V-form seems to be closed; and as a result, the models of the chymotrypsin and methylamine derivatives are more compact than that of the native protein.

Correlation between the human and porcine complement system: a small-angle scattering study of cross immunity and methylamine-induced conformational changes of porcine C3 and C4 proteins

The porcine complement proteins C3 and C4 have been isolated and then characterized using small-angle scattering methods. Within the limits of experimental errors, the porcine proteins are virtually identical with the corresponding human proteins as measured in terms of mol. wt, Mr and radius of gyration, R,: Mr(C3) = 198,000, Mr(C4) = 207,000, and R(C3) = 4.4 nm, R(C4) = 4.5 nm. The C3 and C4 proteins from pigs show cross-immunity with monoclonal antibodies (mAbs) raised against human C3 and C4, respectively. Using the Fab fragments of these mAbs as markers, it is indicated that porcine C3 and C4 undergo a conformational change after reaction with methylamine. The relatively large increase in the radius of gyration observed, delta R = 1.0-1.2 nm, going from the Fab complexes to the Fab complexes of the methylamine derivatives, is similar to that observed for human C3 under similar conditions. This may indicate that methylamine cleaves a labile thiol ester bond supposed to be present within the porcine proteins and that the epitopes interacting with the Fab fragments are very similar to those of the human proteins. Porcine C3 also resembles the human analogue by forming dimers after being subjected to methylamine and dilute lauryl sulphate: Mr = 404,000 and R = 7.9 nm.

X-ray scattering study of hagfish protease inhibitor, a protein structurally related to complement and alpha 2-macroglobulin

A protease inhibitor from hagfish blood plasma, homologous to human alpha 2-macroglobulin, has been studied in solution using small-angle X-ray scattering; the radius of gyration, R, was found to be 7.0 nm, the molecular weight 340,000 +/- 20,000, and the largest distance within the molecule, Dmax, 22 nm. When the inhibitor reacts with chymotrypsin, its 1:1 chymotrypsin complex is found to be more compact than the native molecule, R = 6.1 nm. A very similar conformational change is observed after the protein is reacted with methylamine. The data are consistent with models consisting of two equal elliptic cylinders with the same size as the one used as a model for the complement proteins C3 and C4 [cf. Osterberg et al. (1989) Eur. J. Biochem. 183, 507-511]. In the model for the native protein, these cylinders are arranged in an extended form, and in the one for the methylamine derivative (or chymotrypsin complex), they are closer together so that the projection of their elliptic surfaces forms an angle of about 70 degrees. These models for the hagfish protease inhibitor were expanded to models for the twice as large human alpha 2-macroglobulin using symmetry operations, and the resulting alpha 2-macroglobulin models were found to agree with those emerged from earlier studies involving electron microscopy and X-ray scattering methods.

Molecular modeling of human complement component C4 and its fragments by X-ray and neutron solution scattering

The solution structures of human complement component C4 and five derived fragments, C4u, C4(a + b), C4b, C4c, and C4d, were analyzed by synchrotron X-ray and neutron scattering. The X-ray radii of gyration RG for C4, C4u, and C4(a + b) in H2O buffers are similar at 5.23-5.28 nm, and likewise the cross-sectional radii of gyration RXS are similar at 2.48-2.52 nm. Molecular mass calculations using X-rays and neutrons show unexpectedly that C4c is dimeric; however, all the other forms are monomeric. C4c2 has an X-ray RG of 5.18 nm and an RXS of 2.89 nm. Neutron contrast variation gives RG values at infinite contrast of 4.87-4.93 nm for C4 and C4u, 4.79 nm for C4b, 4.94 nm for C4c2, and 2.69 nm for C4d. The RXS values at infinite contrast are 2.23-2.25 nm for C4 and C4u, 1.89 nm for C4b, and 2.62 nm for C4c2. These data show that a large conformational change occurs on going from C4 to C4b, but not on going from C4 to C4u, and this is attributed to the presence of the C4a moiety in C4u. Comparisons of the C4 and C4u scattering curves show that these are very similar out to a nominal resolution of 4 nm. Scattering-curve models were developed to account for the neutron scattering curves of C4, C4c2, and C4d in 2H2O buffers. The C4c monomer could be represented by a lamellar ellipsoid of size 8 nm x 2 nm x 18 nm. C4d was found to be 4 nm x 2 nm x 9 nm. The combination of these structures gave good accounts of the neutron data for C4, C4b, and C4c2 to resolutions of 5-6 nm. The C4 model was obtained by placing the long axis of C4d parallel to that of C4c such that the cross section is extended. C4b was best modeled by repositioning C4d relative to C4c such that this cross section becomes more compact. The C4 and C4b models are compared with possible structures for the C1 component of complement to show the importance of the surface accessibility of the protease domains and short consensus repeat domains in C1 for C4 activation.

Large intramolecular movement in human complement protein C3 induced by methylamine. A small-angle scattering study using monoclonal antibodies as markers

The reaction of methylamine with complement protein C3, which involves cleavage of a labile thiol ester bond, yields a large intramolecular rearrangement. This is shown by small-angle neutron and X-ray scattering using a Fab antibody as a marker. For the C3(Fab) 1:1 complex, the methylamine reaction yields an increase in the radius of gyration, R, from 4.6 nm to 6.0 nm. In the absence of Fab the corresponding R values increase from 4.4 nm to 5.1 nm. It is estimated that the methylamine-induced increase in R may correspond to a movement of the epitope to a position 5 nm away from the centre of gravity of the C3 molecule. In agreement with this finding, the maximum distance within the C3(Fab) complex increases from 16 nm to 22 nm as a result of the methylamine reaction. In order to explain this conformational change, it is tentatively suggested that the methylamine-induced cleavage of the C3 thiol ester bond leads to a domain rotation within the C3 molecule. In agreement with this idea, the data is consistent with a model which enables a globular domain within the molecule to rotate without redistributing the molecular mass more than that corresponding to the radii of gyration observed.