Small-angle neutron scattering studies of the conformation of myeloma protein MOPC315 and its Fab fragment, and the interaction with a monovalent dinitrophenyl hapten

The first small-angle scattering study of an immunoglobulin A is reported. Neutron measurements have been made to determine conformational parameters of the mouse myeloma protein MOPC315 and to relate these to previous immunoglobulin G results. Use of the contrast method shows that the MOPC315 IgA molecule is not simply globular, that it has a dry volume of 220.0 ± 4.5 nm 3 corresponding to a mass density of 1.275 ± 0.025 g cm −3 and that its full and cross-sectional radii of gyration, corrected for con­centration dependence, are 7.97 ± 0.07 nm, 2.40 ± 0.08 nm and 1.33 ± 0.07 nm respectively. Similar study of its Fab fragment gives a dry molecular volume of 69.0 ± 0.7 nm 3 , a mass density of 1.285 ± 0.015 g cm −3 and un­corrected radii of gyration that are consistent with those of the parent and support an overall ‘ T ’ or ‘ Y ’ conformation in solution. Addition to satu­ration of a small monovalent dinitrophenyl hapten leaves the dry volume of the whole molecule unaltered, but may slightly lower one or more of its radii of gyration. The significance of this finding is discussed. Comparative studies with rabbit anti-dinitrophenyl immunoglobulin G antibody suggest a different initial conformation but similar consequences of hapten binding, which, if real, are probably unrelated to classical complement fixation.