1

intracellular?

Figure 1 Structure of HCD proteins. Except when stated (in vitro for Omm in whom the native protein is known from biosynthesis experiments, and 'intracellular?' for protein sequences deduced from mRNA sequences), the structures shown are those of the serum proteins.

entirely or most often partly (leaving the 5' end of V and the 3' end of JH) deleted, and its 3' flank bears insertions and deletions. Part of the switch region is removed by one or two deletions, one of which encompasses CH1 (entirely or partially). These abnormalities lead to various mRNA splicing patterns removing the possible residual 3' end of Cnl. Entire free HCs are not secreted. They are retained and degraded in the endoplasmic reticulum due to an interaction with chaperone proteins such as the immunoglobulin binding protein BiP which occurs through the CHI domain of the HC. As most HCD proteins lack CHI, they escape the interaction with BiP. As for those rare cases of p-HCD with a deletion of the V domain only, normal LCs are produced which are noncovalently associated with the HC intracellularly, preventing interaction with BiP. Insertion of DNA of unknown origin is constant, usually at the location of deletions. In three genes such insertions include an exon. The unknown sequences present in the mature transcripts are absent from the serum protein in the two cases studied, probably due to amino-terminal proteolysis. In the nonsecretory a-HCD SEC, a further genomic deletion removes the polyadenylation signal of secreted a chains, forcing plasma cells to produce membrane a chains only.

Although LC - mostly k - gene rearrangement seems to be constant, there is usually no detectable LC synthesis in y- and a-HCD. In the single such 7-

HCD case studied, a productive rearranged k gene bore numerous mutations in the VJ region, some of which altered both splice sites of the VJ exon, with direct splicing from the leader exon to Ck. A deletion and two insertions were found in the 3' flank of VJ. These LC abnormalities are reminiscent of those of the HC genes.

See also: Immunoglobulin genes; Immunoglobulin structure; Lymphoma.

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