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.
Department of Chemistry and Molecular Biology, Swedish University of Agricultural Sciences, Uppsala, Sweden.
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R
Osterberg
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U
Nilsson
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T
Stigbrand
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J
Kjems
D000911:Antibodies, Monoclonal; D003176:Complement C3 / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008744:Methylamines / Q000494:pharmacology*; D009502:Neutrons; D011487:Protein Conformation; D012542:Scattering, Radiation; D014961:X-Ray Diffraction
D000911:Antibodies, Monoclonal; D003176:Complement C3; D008744:Methylamines; C027451:methylamine