Solution Structure of Human Secretory Component and Implications for Biological Function

Abstract

Secretory component (SC) in association with polymeric IgA (pIgA) forms secretory IgA, the major antibody active at mucosal surfaces. SC also exists in the free form, with innate-like neutralizing properties against pathogens. Free SC consists of five glycosylated variable (V)-type Ig domains (D1-D5), whose structure was determined by x-ray and neutron scattering, ultracentrifugation, and modeling. With a radius of gyration of 3.53-3.63 nm, a length of 12.5 nm, and a sedimentation coefficient of 4.0 S, SC possesses an unexpected compact structure. Constrained scattering modeling based on up to 13,000 trial models shows that SC adopts a J-shaped structure in which D4 and D5 are folded back against D2 and D3. The seven glycosylation sites are located on one side of SC, leaving known IgAbinding motifs free to interact with pIgA. This work represents the first analysis of the three-dimensional structure of fulllength free SC and paves the way to a better understanding of the association between SC and its potential ligands, i.e. pIgA and pathogenic-associated motifs.The mucosal epithelia lining the gastrointestinal, respiratory, and urogenital tracts represent the largest surface (400 m 2 for the human body) in contact with the external environment (1). Consequently, they are permanently exposed to foreign antigens and must therefore be efficiently protected to prevent possible damage by pathogenic agents (2). In response to this threat, a major immune defense system is mediated by secretory immunoglobulin A (SIgA) 5 within the lumen (3), where SIgA exerts its function of antigen neutralization. IgA is produced locally as polymeric IgA (pIgA), which consists of two or four IgA molecules linked covalently by a J chain (4). Subsequent transport of pIgA (and to a lesser extent pentameric IgM) across the epithelium is ensured by the polymeric Ig receptor (pIgR), which is expressed on the basolateral surface of epithelial cells. Following cleavage at luminal surfaces, SIgA is released as a complex of pIgA and the cleaved extracellular portion of the pIgR which is termed secretory component (SC). Because secretory IgM can be present in external secretions with an accessory role as a neutralizing antibody, this suggests that SC has a generic antibody-binding ability, being able to bind to both pIgA and pentameric IgM (5). This selectivity may be required to allow only larger, polymerized antibodies protected by heavily glycosylated SCs to enter the harsh environment of external secretions.SC consists of five immunoglobulin-like domains (D1-D5) and up to seven glycan chains. SC bound to pIgA protects the antibody against proteolytic digestion (6) and governs anchoring of SIgA at mucosal surfaces (7). Free SC is also found in secretions and is now recognized as an active antibacterial participant to protect mucosal surfaces against Helicobacter pylori (8), enteropathogenic Escherichia coli and Clostridium difficile toxin A (9), and Streptococcus pneumoniae choline binding protein A (10, 11). Recombinant human SC produced fro...

Methods to determine the interactions of micro- and nanoparticles with mucus

“…SANS was also used in combination with small-angle X-ray scattering (SAXS) to determine the structure of pig gastric mucin [63], the chain dimensions determined from this study were significantly different to previous studies, i.e. radius c.a.…”

“…When investigating relationships between physical properties and molecular structure, the determination of molecular organization within complex systems is fundamental; small-angle neutron scattering (SANS) -a powerful, non-destructive technique that probes organizational structure on a 1-200 nm lengthscale -can be easily applied for the study of delicate biological materials, although most studies published to date have focused on mucin [61][62][63][64][65].…”

Constrained solution scattering modelling of human antibodies and complement proteins reveals novel biological insights

“…First, we determined structures for monomeric IgA1 and IgA2 (the two human subclasses of IgA; Boehm et al 1999;Furtado et al 2004), then this was followed by structures for the dIgA1 dimer and the five-domain structure of recombinant SC (Bonner et al 2007. Finally, the structures of SIgA1 and SIgA2 were elucidated (Bonner et al 2009a,b).…”

“…In addition, a flat neutron background arises from the incoherent scatter from non-exchangeable macromolecular protons or residual solvent protons. This requires a uniform correction of 0.5 -2.7 per cent of I(0) at all Q-values (Furtado et al 2004;Bonner et al 2007). …”

Secretory IgA-Mediated Neutralization of Shigella flexneri Prevents Intestinal Tissue Destruction by Down-Regulating Inflammatory Circuits

“…Radiolabeled SIgA was obtained as previously described (10). To visualize its localization, 10 6 cpm were administered per ileal loop, corresponding to 7 g of [ 35 S-Cys, 35 S-Met]-IgAC5 mixed to 97 g of unlabelled IgAC5, then reconstituted with recombinant hSC.…”