A new extra sequence at the amino terminal of a μ heavy chain disease protein (DAG)

The primary structure of a human μ heavy chain (DAG) protein is described. The native protein is a circular decamer with a molecular weight (Mr) of 500 kDa, each decamer being constituted of the constant domainsCμ2, Cμ3 and Cμ4 and interlinked by 15 disulfide bridges. At its NH2-terminal each monomeric chain starts with an “extra sequence”. The amino acid sequence of this segment is Arg-Gln-Ser-Asp-Asp-Pro-Val-Leu-Arg-Gly-Thr-Thr-Val-Pro-Val-Thr-Glu and its reinitiation point is located at Val223 (Gal numbering), at the beginning of Cμ2. This sequence has no homology with any other protein included in the present databases.