Abstract
Immunoglobulin M formed by mouse plasmacytoma MOPC 104E was examined by electron microscopy of negatively stained material. The protein displayed a central core to which were attached five and very occasionally six, radially arranged subunits. The subunits were Y-shaped with the branches of the Y presumably representing the two Fab fragments per 7S subunit. The distance from the centre of the molecule to the point of branching was 100–110 Å, and the length of each branch was 60–80 Å. IgM subunits (obtained by partial reduction of IgM with dithioerithritol) could be reconstituted after removal of the sulphydryl reagent. The reconstitution was confirmed by electrophoresis on polyacrylamide gel and by electron microscopy; assembly of molecules with the characteristic radial symmetry was observed.
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