Fragmentation and reduction of bovine secretory component. Preparation of a biologically active fragment and some evidence for a multiple-domain structure

Domain deletions in the human polymeric Ig receptor disclose differences between its dimeric IgA and pentameric IgM interaction

The human polymeric Ig receptor (pIgR), or transmembrane secretory component, is basolaterally expressed on secretory epithelial cells; its function is to transport externally J chain-containing dimeric IgA and pentameric IgM. The ligand-binding extracellular part of this receptor contains five disulfide-stabilized domains which show considerable homology with the variable domains of Ig chains. The N-terminal domain 1 (D1) mediates the initial noncovalent ligand interaction. In this study we made deletions of the human pIgR D2 and D3 (pIgRDelta2,3), or D4 and D5 (pIgRDelta4,5), to investigate the influence of these domains in receptor binding and transport of dimeric IgA and pentameric IgM across MDCK cells transfected with the truncated receptors. Both mutants were found to bind pentameric IgM, but only pIgRDelta4,5 bound dimeric IgA. These results showed that the two ligands interact differently with human pIgR; binding of pentameric IgM apparently depends fully on strong interactions with D1, while binding of dimeric IgA in addition depends on elements within D2 and / or D3 to support the initial noncovalent binding to D1. Moreover, our studies imply that dimeric human IgA binds differently to pIgR from various species. This observation cautions against interpretation of functional studies performed with non-homologous receptor-ligand pairs.

Tryptic digestion of bovine secretory IgA at elevated temperature and in urea. Isolation of SC domain 1 which is covalently bound to IgA dimer and binds non-covalently to IgM

1. Tryptic cleavage sites in bovine secretory component (SC) which become inaccessible when SC is bound to IgA dimer remained inaccessible at 60 degrees C and in 4 M urea at 37 degrees C. 2. This suggests the presence of strong interactions compatible with published affinity constants of ca 10(8) M-1. 3. In 5 M urea at 37 degrees C further cleavage of bound SC did occur to produce a fragment consisting of domain 1 which was disulphide bridged to the IgA dimer. 4. Binding studies on the isolated fragment showed that domain 1 did not account for all the binding by SC. 5. Cleavage of the isolated fragment with iodosobenzoic produced a smaller fragment consisting of the n-terminal third of domain 1 (residues 1-35). This N-terminal fragment showed significant binding.

Cyanogen bromide cleavage of bovine secretory component and its tryptic fragments

1. Five-domain bovine secretory component and its two-domain and three-domain tryptic fragments have been treated with cyanogen bromide. 2. N-terminal sequence analysis of the purified products showed that cleavage occurred within the disulphide bridged polypeptide loop of domain 2. The site lies within the region that binds IgM and IgA dimers. 3. The relative binding of the CNBr fragments to IgM has been measured and indicates that domains 1 and 3 are directly involved. 4. A possible role for domain 2 is less clear and domains 4 and 5 do not participate in binding.

The structure of bovine secretory component

Bovine secretory component (SC) has been cleaved with trypsin into a series of fragments and their N-terminal amino acid sequences have been determined. The close homology with the known sequence of human SC has enabled the sequential order of the fragments to be deduced. The results indicate that bovine SC consists of a single glycosylated polypeptide chain (Mr 74,000) folded into five globular immunoglobulin-like domains. A protein (Mr 94,000) has been isolated from detergent solubilised bovine epithelial membranes from liver, intestine and mammary gland. This membrane protein is specific for the binding of J-chain linked IgM and IgA dimers. It can be proteolytically cleaved into a water soluble SC-like portion and a detergent soluble hydrophobic portion. Bovine SC is therefore most likely to be the extracellular part of an epithelial receptor which mediates the transport of IgA dimers to mucosal surfaces. The various tryptic fragments from bovine SC have been shown to differ in their relative binding affinities for IgM and IgA dimers. The results imply that the first three domains of bovine SC are most involved in binding and domains 4 and 5 play subsidiary roles. Computerized prediction and modelling methods have been used to deduce possible tertiary and quaternary structures for SC. There are good indications that the molecule has an elonaged "zig-zag" structure stabilized by longitudinal inter-domain contacts. A model of SC bound to IgA dimer is presented.

The sites of tryptic cleavage in bovine secretory component: structural and functional implications

Tryptic fragments from bovine secretory component and sIgA have been separated by HPLC and/or SDS polyacrylamide gel electrophoresis and electroblotting. Their N-terminal amino acid sequences have been determined and their positions in the secretory component molecule deduced by homology with the amino acid sequences of human secretory component and rabbit polyimmunoglobulin receptor. Taken in conjunction with the known binding affinities of the tryptic fragments, the results imply that the three most N-terminal domains of secretory component are directly involved in binding IgM and IgA dimers. The results also favour the concept of an extended 'zig-zag' structure for the secretory component molecule.

The transport and metabolism of bovine IgM

IgM is present in cows milk, is able to bind secretory component (SC), has a purported role as a secretory immunoglobulin in other species and has been identified with various antibody functions in cows milk. To determine the origin of cows milk IgM, we administered extrinsic 131I-IgM to lactating cows with cannulated bile and parotid ducts and studied the kinetics of its disappearance from serum and its appearance in milk, bile and parotid saliva for 60 hr post-injection. Pentameric IgM appeared to require a long equilibration time and disappeared from serum with a T1/2 of 40 hr. The transport of IgM into bile also appeared biphasic. Results showed that no 131I-IgM was transported intact into parotid saliva and that most radioactivity in milk and bile after 6 hr was in the form of low mol. wt, TCA-precipitable fragments rather than of the size of a pentamer. During the first 24 hr only 0.83% of the administered dose reached the milk in pentameric form, nevertheless, isotope dilution calculations indicated that nearly all milk IgM was derived from serum. During a 12 hr collection period, corresponding to one milking, greater than 200 mg of serum IgM is secreted in milk. During the first 24 hr, only 0.70% of the administered IgM reached the bile as a pentamer. It was calculated that 50% of the pentameric IgM in bile, 3 hr after administration, was serum-derived. Twenty-five per cent of the IgM appearing in bile and ca 10% of the IgM appearing milk, becomes associated with secretory component. A hypothesis to explain the degradation associated with this inefficient transport mechanism is presented.

Human secretory component. IV: Antigenic regions involved in in vitro binding to dimeric IgA

The aim of this report was to identify the region(s) of the secretory component (SC) molecule involved in in vitro binding to dimeric IgA. Inhibition of the SC binding was tested by Fab' antibody fragments directed against the accessible (A) and inaccessible (I) regions of SC. Antibodies directed against the main 38.5-kDa trypsin fragment of SC, and antibodies from two immune sera with a wide anti-SC spectrum, were also used. The specificity and activity of the five antibody preparations were established by double-diffusion in gel and by their SIgA combining capacity. Inhibition curves were established by RIA using constant amounts of 125I SC, dimeric IgA and increasing quantities of the various Fab' antibodies. These results indicated involvement of a larger part than the I region of the SC molecule in combination with dimeric IgA, perhaps including a second (minor?) site of binding on the accessible parts in addition to the major region located on I.

A bovine epithelial membrane protein that binds polymeric immunoglobulin and has a structure related to that of bovine secretory component

A protein of Mr 94000 was isolated from detergent-solubilized bovine intestinal, liver and mammary-gland membranes. It binds immunoglobulin M and also undergoes proteolytic fragmentation in a similar manner to bovine secretory component (Mr 74000). The affinity-purified membrane protein is therefore most likely to be the bovine epithelial receptor for polymeric immunoglobulin. A structural model is proposed.

Differences in fragmentation between bound and unbound bovine secretory component suggest a model for its interaction with polymeric immunoglobulin

Unbound bovine secretory component was cleaved into two-domain and one-domain fragments by trypsin within 1 h. Bovine secretory component covalently bound to bovine IgA dimer, as in secretory IgA, was much more resistant to fragmentation, which did not proceed beyond the three-domain stage even after 5 h. Bovine secretory component non-covalently bound to bovine IgM or to human IgM or IgA polymer was also relatively resistant to fragmentation, which again was largely arrested at the three-domain stage. A model for the binding of secretory component to polymeric immunoglobulin is proposed.