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

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Abstract
The 1st small-angle scattering study of IgA is reported. Neutron measurements were made to determine conformational parameters of the mouse myeloma protein MOPC315 and to relate these to previous IgG 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 nm3 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, 2.40 .+-. 0.08 and 1.33 .+-. 0.07 nm, respectively. Similar study of its Fab fragment gives a dry molecular volume of 69.0 .+-. 0.7 nm3, 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 IgG antibody suggest a different initial conformation but similar consequences of hapten binding, which, if real, are probably unrelated to classical complement fixation.