PRIMARY STRUCTURE OF A MONOCLONAL IGA-IMMUNOGLOBULIN (IGA TRO) .2. AMINO-ACID SEQUENCE OF H-CHAIN, ALPHA-TYPE, SUBGROUP-3 - STRUCTURE OF COMPLETE IGA-MOLECULE
Sequence studies on human immunoglobulin A were carried out with the isolated L-chain, the isolated H-chain and with BrCN-fragments of the H-chain. Tryptic and chymotryptic peptides were prepared from the totally reduced and alkylated .alpha.-chain or from BrCN-fragments. The variable part comparised positions 1-119, the constant part [CP] residues 120-472. According to its homology with other variable parts, .alpha.-chain Tro. clearly belongs to subgroup III of the H-chains. The CP is composed of 3 homology regions (C1-C3) which originated from a common ancestor by repeated gene duplications early in evolution. Each homology region corresponds in its length and its sequence to the CP-region of the L-chains. The hinge-region, which connects the C1- and the C2-region, originated from the carboxy-terminal end of the C1-region by a 2-fold partial gene duplication comprising 8 amino acids. The C3-homology region is terminated by an additional carboxy-terminal piece of 18 residues. The .alpha.-chain contains 17 cysteine residues, 8 of which form the usual intrapeptidal loop S-S-bridges and 1 the connection between the L- and H-chain. Two additional cysteine residues are located in the C1-region and 5 more in the C2-region, forming intra- and inter-peptidal S-S-bonds.