ULTRASTRUCTURE OF PAPAIN AND PEPSIN DIGESTION FRAGMENTS OF HUMAN IgM GLOBULINS

The ultrastructure of papain and pepsin-digested products of human IgM globulins has been analyzed. Papain digestion was performed both in the presence and absence of cysteine. The Fcµ fragment was found to represent the central ring structure in the intact IgM molecule, plus a minor part of the appendages extending from the ring. The Fcµ ring structure was occasionally seen to be composed of dimers of short rods, probably identical with the endpieces of two µ-chains. Such dimeric structures, released from the intact Fcµ rings, had a tendency to aggregate sidewise, producing complexes of varying size. The dimensions of the Fcµ fragments were: outer diameter approximately 85 A, inner diameter about 40 A. The length of the protrusions varied from 20–30 A. The Fabµ preparations contained long strands of sidewise aggregated, short rod-shaped fragments. No aggregates were seen in the F(ab'')(2)µ preparations. The two Fab''µ units in the dimeric F(ab'')(2)µ fragments were usually parallel to each other. The dimensions of the Fabµ and F(ab'')(2)µ fragments were 50–80 A x 30 A and 75–80 A x 55 A, respectively. These findings provide morphological evidence that the C-terminal ends of the µ-chains (the Fcµ fragment) make up the central ring structure in the IgM molecule. They further indicate that the F(ab'')(2)µ fragments constitute about ¾ of the appendages extending from this ring structure.