Biophysical and Biochemical Characterization of Avian Secretory Component Provides Structural Insights into the Evolution of the Polymeric Ig Receptor

Abstract

The polymeric immunoglobulin receptor (pIgR) transports polymeric antibodies across epithelia to the mucosa, where proteolytic cleavage releases the ectodomain (secretory component; SC) as an integral component of secretory antibodies, or as an unliganded protein that can mediate interactions with bacteria. SC is conserved among vertebrates, but domain organization is variable: mammalian SC has five domains (D1-D5), whereas avian, amphibian and reptilian SC lack the D2 domain, and fish SC lacks domains D2-D4. Here we used double electron-electron resonance spectroscopy and surface plasmon resonance binding studies to characterize the structure, dynamics and ligand binding properties of avian SC, avian SC domain variants, and a human SC variant lacking the D2 domain. These experiments demonstrated that unlike human SC, which adopts a compact or “closed” domain arrangement, unliganded avian SC is flexible and exists in both closed and open states, suggesting that the mammalian SC D2 domain stabilizes the closed conformation observed for human SC D1-D5. Experiments also demonstrated that avian and mammalian pIgR share related, but distinct, mechanisms of ligand binding. Together, our data reveal differences in the molecular recognition mechanisms associated with evolutionary changes in the pIgR protein.

The Structures of Secretory and Dimeric Immunoglobulin A

“…D2 is absent from this bridge, and does not appear to contact the HCs or JC, suggesting that despite its reported contribution to dIgA binding kinetics (Stadtmueller et al, 2016a), its role is indirect. The distal location of D2 in the SIgA structure is also consistent with a model in which SC from birds, reptiles and amphibians, which are all lacking the D2 domain (Stadtmueller et al, 2016b), would bind analogous to D1-D3-D4-D5 in our SIgA structure.…”

Innate immune functions of avian intestinal epithelial cells: Response to bacterial stimuli and localization of responding cells in the developing avian digestive tract

“…FSC is formed by cleavage of unbound pIgR at the apical side of enterocytes [ 31 , 32 ]. To investigate a similar role for the secretory component in the chicken we used secretory component specific immunohistochemistry to study its expression in intestinal tissue; the chicken homologue (GG-pIgR) is bound by anti-human secretory component [ 33 ]. Fig 8 (panel A) shows that the receptor was present in chick IEC with marked and intensive staining in goblet cells and the staining was not confined to the baso-lateral surfaces.…”

The contribution of modern EPR to structural biology

“…Based on the Anfinsen dogma, it became customary to explain function in terms of a single conformation or of well-defined transitions between a few conformations defined at atomic resolution. While this is certainly a reasonable approximation in some cases [75][76][77], availability of distance distributions demonstrates that rather often conformation transitions are coupled to order-disorder transitions or are shifts in disorder equilibria [39,[78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94]. Among the systems addressed by PDS to date, the fraction where at least one state is genuinely disordered is surprisingly large.…”

“…In cardiac myosin-binding protein C, phosphorylation causes compaction and reduces disorder of the Pro/Ala-rich linker between two immunoglobin domains [88]. The human and teleost fish secretory components, which could be crystallized, exhibit well-defined conformations in the absence of their immunoglobulin ligands [95], whereas the unliganded avian secretory component is flexible, pointing to a distinct mechanism of ligand binding of secretory components among vertebrates [91]. Pro-apoptotic Bax exhibits disorder in the piercing domain that anchors Bax in the mitochondrial membrane, which may be related to its ability to form pores of different sizes by varying its degree of oligomerization [39].…”