Immunochemical and biochemical study of a human Fcmu-like fragment (mu-chain disease).

An abnormal protein, from a petient with mu-chain disease has been studied: protein BUR. It is devoid of the F (ab'')2 mu fragment; molecular weight determinations and immunological data identify the protein as a F(c) 5 mu fragment. Similarly, carbohydrate determinations and proteolysis experiments relate it to a Fcmu fragment. Nevertheless, the protein, which tontains J-chain, lacks the entire covalently bonded structure of the F (c)5mu fragment. C-terminal analysis shows a tyrosine residue identical to the C-terminal amino acid of mu-chains. Sequence of the N-terminal region determined for 15 residues shows identity with sequence 338-352 and sequence 333-347 of two entire mu-chains previously studied. The relation of BUR to other heavy chain disease proteins is discussed.