Crystal structure of galactan-binding mouse immunoglobulin J539 Fab at 4.5-A resolution.

An electron-density map of the mouse galactan-binding immunoglobulin J539 (IgA2,kappa) Fab has been calculated to a resolution of 4.5 A by the method of heavy atom isomorphous replacement with four derivatives. The map has been interpreted with the aid of a computer program which systematically searched for the best fit between the electron-density map and the known coordinates of individual immunoglobulin domains. The quaternary structure of J539 Fab at this resolution appears similar to that of another mouse immunoglobulin, IgA2,kappa Fab, McPC603. The model coordinates for J539 Fab should allow us to proceed directly to a high-resolution structure determination without further heavy atom isomorphous replacement.