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.