Fragmentation and reduction of bovine secretory component. Preparation of a biologically active fragment and some evidence for a multiple-domain structure.
A tryptic fragment (A) of Mr 25000 was prepared from bovine secretory component. The fragment binds polymeric immunoglobulin, although 9 times less effectively than secretory component on a molar basis. The fragment has four buried half-cystine residues and two exposed half-cystine residues. It gives rise to two fragments of Mr 11000-13000 on prolonged digestion with trypsin, and these do not bind polymeric immunoglobulin. It is proposed that fragment A consists of two immunoglobulin-like domains. Bovine secretory component was found to have 9-11 buried half-cystine residues and four exposed half-cystine residues. Reduction and alkylation of the exposed residues decreases the binding of polymeric immunoglobulin by 3-fold. Initial tryptic cleavage of bovine secretory component gives a fragment (Q) disulphide-bridged to a further fragment (T). Fragment Q is similar in size to a three-domain immunoglobulin fragment, and fragment T is similar in size to a two-domain immunoglobulin fragment. The two-domain fragment A is derived from fragment Q by further tryptic cleavage. The results are compatible with the proposal by Mostov, Friedlander & Blobel [(1984) Nature (London) 308, 37-43] that secretory component consists of multiple immunoglobulin-like domains. The results also indicate that optimal binding of polymeric immunoglobulin involves several domains stabilized by an exposed disulphide bridge.
D000478:Alkylation; D000818:Animals; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D003553:Cystine / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D007128:Immunoglobulin Fragments; D007075:Immunoglobulin M; D007457:Iodine Radioisotopes; D007460:Iodoacetamide; D008956:Models, Chemical; D010446:Peptide Fragments / Q000032:analysis*; D012635:Secretory Component; D012685:Sepharose; D014357:Trypsin
D007128:Immunoglobulin Fragments; D007075:Immunoglobulin M; D007457:Iodine Radioisotopes; D010446:Peptide Fragments; D012635:Secretory Component; D003553:Cystine; D012685:Sepharose; D014357:Trypsin; D007460:Iodoacetamide