tailpiece-binding residues are solvent acces-
sible (Fig. 6, A and B). Expanding on the hy-
pothesis of Stadtmuelleret al.( 15 ), we propose
a model in which the closed pIgR conforma-
tion solvent-accessible residues make initial
contact with the JC and Fc2 tailpiece. This
leads to a conformational change in the re-
ceptor that breaks the D1–D4–D5 interface
and exposes the remaining contact residues
in D1 to bind Fc1 (Fig. 6C). The open pIgR
conformation is stabilized through a new
D1–D3 interface that creates optimal spacing
between D1 and D5 to enable secondary site
binding to Fc2. Reduction of the D5C468–
D5C502intramolecular disulfide frees D5C468
to form the intermolecular disulfide with
Fc2AC311, locking pIgR into a bent, pIgA-bound
conformation (fig. S8). When pIgR is proteo-
lytically cleaved upon transcytosis, SC remains
covalently attached to pIgA, leading to the re-
lease of sIgA at the mucosa.
Both the SC and pIgA are modified with
N-linked glycans, which not only facilitate
proper protein folding, but also mediate the
attachment and neutralization of pathogens
( 1 , 8 ). Our structures revealed ordered N-linked
glycans on JCN49,FcN337,FcN459,SCN65,SCN72,
SCN168,SCN403,SCN451,andSCN481,whichare
not in contact with the protein and are placed
away from any SC–pIgA interaction interfaces
(fig. S9). These glycosylation sites could be
relevant in facilitating host and pathogen lec-
tin binding.
Extensive mutational analysis has previously
been performed on both pIgA and the pIgR,
assessing both binding and transcytosis. As
seen in our structures, the C-terminal 25 resi-
dues of the JC are critical for SC binding, where-
as JCC15and JCC69are important for efficient
transcytosis, likely due to stabilization of the
pIgA assembly through their covalent bonds
to the Fc tailpieces ( 16 ). Mutagenesis of the
human SC D1 residues N30, R31, H32, and R34
in the highly homologous rabbit SC either sig-
nificantly reduced or abolished SC binding to
human pIgA ( 17 ), consistent with the identifi-
cation of these as key interaction residues in
our structures. Additionally, swapping of the
rabbit SC D1 CDR2 or CDR3 loops with the
corresponding structurally similar yet sequence
diverse CDRs from D2 also abolished binding
( 17 ), consistent with both CDRs making impor-
tant contacts to pIgA in our structures. Muta-
tion of FcC311,FcP440–F443, or deletion of the
Fc tailpieces results in a significant impair-
ment in transcytosis ( 18 ), a functional conse-
quence highlighting the important roles these
regions play in SC binding, as observed in
our structures. Taken together, these muta-
genesis studies underscore the functional
importance of the interactions described in
our structures.
The coreceptor Mac-1 (CD11b/CD18) is re-
quired to bind and activate FcaRI signaling
by sIgA but not dimeric IgA, possibly due to
steric hindrance by SC ( 12 , 19 ). Our sIgA dimer
structure showed that JC binding to the Fcs
overlapped with two of the four FcaRI-binding
sites (fig. S7). Similarly, in the tetramer, only
two sites were available because the Fc–Fc-
mediated contacts also occurred at the Ca 2 – Ca 3
interface. We saw no evidence of additional
steric hindrance between the SC and the two
accessible FcaRI-binding sites in the dimer
or tetramer. In the pentamer, no FcaRI-binding
sites were accessible, suggesting that pentame-
ric IgA cannot bind the receptor. Because the
oligomeric state of sIgA used in the Mac-1 studies
is unknown ( 19 ), it is difficult to interpret the
requirement for Mac-1 in FcaRI signaling.
Although many recombinant IgGs have been
approved for therapeutic use in fields such as
oncology, immunology, and ophthalmology, an
IgA-based therapeutic has yet to be approved,
likely in part due to the short serum half-life ofthis isotype ( 9 ). The structural characterization
of sIgA provided here opens the possibility for
engineering half-life extension properties into
pIgA while maintaining its fundamental muco-
sal tissue–targeting properties. Furthermore,
using the structural information as a frame-
work, recombinant sIgA could be designed
for potential oral delivery of a therapeutic
antibody because the SC is known to provide
stability and protection to pIgA from proteo-
lytic degradation ( 8 ). Our studies also raise
the question of whether the JC-containing
pentameric IgM is formed through a similar
oligomerization mechanism and if pIgR bind-
ing is analogous. High sequence conservation
between IgA and IgM tailpieces (fig. S5B) sug-
gests a shared mechanism of polymerization.
However, the presence of an additional con-
stant domain, Cm4, in IgM may require a mod-
ified mode of pIgR recognition. Additional
structural studies will be needed to addressKumaret al.,Science 367 , 1008–1014 (2020) 28 February 2020 6of7
Fig. 6. Model for pIgA recognition by pIgR.(A) Structure of the free SC (PDB 5D4K) in a closed
conformation stabilized by a D1–D4–D5 interface. CDR residues of D1 involved in pIgA binding are colored
green and their side chains are shown as sticks. (B) Magnification of the box in (A) highlighting pIgA contact
residues of D1 that are solvent accessible or buried. (C) Model of the conformational change in pIgR that
converts it from the closed, unbound state (left, free SC) to the open, pIgA-bound state (right, dimer
structure aligned to D1 of free SC).RESEARCH | RESEARCH ARTICLE