Biosynthetic and structural studies of a heavy chain disease protein.

A new heavy chain disease protein ((gamma)HCD-JM) has been characterized by antigenic and structural criteria. The protein belongs to the IgG3-subclass and is closely related to Fc-fragment of G3-immunoglobulins. The predominant N-terminal amino acid of this protein is glutamic acid in the uncyclized form, and that of another (gamma)HCD is glycine. Studies of the N-terminal peptides indicate that the N-terminal portion of the (gamma)3-heavy polypeptide chain is absent from the (gamma)HCD-JM. These findings rule out a process of normal heavy chain initiation and a large deletion of the Fd region as being responsible for these two heavy chain disease proteins. The (gamma)HCD-JM is a secretory product of cells from bone marrow as shown by studies of in vitro incorporation of amino acids-(14)C. Bone marrow and lymph node have a population of lymphoplasmacytic cells which by immunofluorescence contain (gamma)-heavy chain antigens in the absence of light chain antigens.