Biophysical and Biochemical Characterization of Avian Secretory Component Provides Structural Insights into the Evolution of the Polymeric Ig Receptor
The polymeric Ig receptor (pIgR) transports polymeric Abs across epithelia to the mucosa, where proteolytic cleavage releases the ectodomain (secretory component [SC]) as an integral component of secretory Abs, 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. In this study, 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 (hSC) variant lacking the D2 domain. These experiments demonstrated that, unlike hSC, 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 hSC 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.
© 2016 by The American Association of Immunologists, Inc. Received for publication March 17, 2016. Accepted for publication June 11, 2016. Published online before print July 13, 2016. This work was supported by National Institute of Allergy and Infectious Diseases Grant AI04123 (to P.J.B.), a Cancer Research Institute Irving Postdoctoral Fellowship (to B.M.S.), a Jules Stein Professorship Endowment (to W.L.H.), and National Institutes of Health Grant EY005216 (to W.L.H.). We thank Jost Vielmetter and the Caltech Protein Expression Center for assistance with protein expression.We also thank members of the Bjorkman and Hubbell laboratories for insightful discussions. The authors have no financial conflicts of interest.
Accepted Version - nihms796559.pdf
Supplemental Material - JI_1600463_Supplemental_Figures_1.pdf