title: The thiol groups of mouse immunoglobulin A. Incomplete formation of the C alpha 1-domain disulphide bridge.
abstract: The BALB/c IgA (immunoglobulin A) myeloma protein M167 contained on average 5.7 free SH groups per IgA dimer. These groups were preponderantly on the heavy chains and comprised two distinct populations: 3.3 exposed SH groups per dimer in the Fc region, and 2.4 buried SH groups per dimer in the Fd region, detectable o only after denaturation. To locate the cysteine residues involved, labelled peptides were purified from thermolysin digests of radioalkylated IgA by high-performance liquid chromatography. From the amino acid compositions of the peptides, the exposed thiol groups were assigned to Cys-307 in the C alpha 2 domain, which thus existed in the reduced form to an extent exceeding 80%. This residue may allow attachment of secretory component to dimer IgA in the mouse to proceed via thiol-disulphide exchange. The buried thiol groups were assigned to Cys-150 and Cys-208, in the C alpha 1 domain, each being in the reduced form to the extent of approx. 30%. This pair of residues would normally give rise to the characteristic intradomain disulphide bridge. It appears that disulphide formation is not a crucial event during folding of the C alpha 1 domain in IgA biosynthesis. The sequence in the region 140-151 was re-investigated, and residue 142 was shown to be serine, not cysteine, helping explain the lack of heavy-chain-light chain bonding in BALB/c mouse IgA. A disulphide-bond model for mouse IgA is proposed on the basis of these assignments and other features of the mouse alpha-chain sequence.
mesh_terms: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D004220:Disulfides / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D005033:Ethylmaleimide; D007070:Immunoglobulin A; D007460:Iodoacetamide; D051379:Mice; D008807:Mice, Inbred BALB C; D008956:Models, Chemical; D009194:Myeloma Proteins; D010446:Peptide Fragments / Q000032:analysis; D013438:Sulfhydryl Compounds / Q000032:analysis*
chemical_list: D004220:Disulfides; D007070:Immunoglobulin A; D009194:Myeloma Proteins; D010446:Peptide Fragments; D013438:Sulfhydryl Compounds; D005033:Ethylmaleimide; D007460:Iodoacetamide