of IgA to the Fc core and the addition of the SC
would result in the formation of a more rigid
molecule amenable to high-resolution structure
determination. In humans, there are two IgA
isotypes, IgA1 and IgA2. Both isotypes can
form dimers and higher-order oligomers,
though there is a propensity for recombinant
IgA1 to form dimers and for IgA2m2 to form
higher-order oligomers ( 9 ). Therefore, we co-
expressed human IgA1 or IgA2m2 Fc with JC
to form dimers or tetramers and pentamers,
respectively (figs. S1 and S2), and subsequently
assembled them with the SC. Single-particle
cryo-EM was used to determine the structures
of the resulting dimeric, tetrameric (two classes),
and pentameric sIgAs at 2.9-, 3.0-, 2.9-, and
3.0-Å resolution [Fourier shell correlation
(FSC) = 0.143], respectively (figs. S1 to S3 and
table S1). Two classes were obtained for the
tetramer; the Fcs were slightly bent toward
each other in class 1 and were more planar in
class 2 (fig. S2). The difference was subtle and
did not affect the pIgA/SC interaction inter-
face. The local resolution was highest at the
central core, which contains the key interac-
tion interfaces (fig. S4).
Overall architecture of sIgA
sIgA1 adopted a tail-to-tail planar dimer with
the two Fcs positioned at an ~110° angle and
held tightly by the JC, which functioned as a
clasp (Fig. 1). The SC made extensive contacts
with both Fcs and the JC, binding diagonally
across the ~50° gap between the two mono-
mers. The structure of the core sIgA1 dimer
was relatively unchanged in the sIgA2m2 tet-
ramer and pentamer, consistent with the 94%
sequence identity between the Fcs of the two
isotypes (fig. S5). Therefore higher-order poly-
mers are assembled through incorporation
of two or three additional Fcs in-plane to the
original dimer (Fig. 2). The pentamer adopted
an asymmetric pentagon containing an ~50°
gap between Fc1 and Fc2 (Fig. 2), reminis-
cent of the JC-containing IgM pentamer ( 10 ).
The SC bound at the gap, cross-bridging two
Fcs in a manner similar to that of the IgM-
binding apoptosis inhibitor of macrophage pro-
tein ( 10 ), suggesting a common binding mode.
Structure of the JC
Weusedthe2.9-ÅmapofthesIgA1dimer
(fig. S6) to de novo build a model containing
all but six residues of the 137–amino acid JC
(residues 5 to 94 and 97 to 137), as well as the
tailpieces of each Fc. The JC was almost en-
tirely composed ofb-sheets and loops (Fig. 3,
A and B), consistent with secondary structure
predictions and circular dichroism studies
( 11 ). Previous biochemical studies identified
intramolecular disulfides between JCC13and
JCC101,JCC72and JCC92,andJCC109and JCC134,
and intermolecular disulfides between JCC15and
Fc2C471and JCC69and Fc1C471( 3 ). Our struc-
ture confirmed these disulfides (Fig. 3C) and
additionally revealed a much larger network
of noncovalent interactions. The JC intricately
joined the two Fcs of the dimer core though its
threeb-hairpins and four-stranded, twisted
b-sandwich. It lay at an angle bridging the
top of Fc2 (defined as the SC-binding face)
throughb-hairpins 1 and 2, to the bottom of
Fc1 throughb-hairpin 3, creating an asymmetric
dimer (Fig. 3A). The JCb-hairpins made pre-
dominantly hydrophobic interactions withthe two Fcs (Fig. 3D).b-hairpins 2 and 3 in-
teracted with the same region of Fc2 and Fc1,
respectively, with theirb-strands packing
against FcM433and FcF443and their loops
contacting FcL258(Fig.3D).ThissameFc-
interaction site is also exploited by the human
Fcareceptor I (FcaRI) and theStaphylococcus
aureusSSL7 toxin ( 12 , 13 ), suggesting a hot
spot for Fcarecognition (fig. S7). The dimer
was further stabilized through interaction of
the Fc tailpieces with the JC, where Fc1 andKumaret al.,Science 367 , 1008–1014 (2020) 28 February 2020 2of7
ACBDFig. 2. Cryo-EM structures of tetrameric and pentameric sIgA2m2.(A) Top and back view schematics
of subunit arrangements in the tetramer. (B) Cryo-EM reconstruction of tetrameric sIgA2m2. Transparent
maps overlaid with the model are shown. (C) Same as in (A) but for pentameric sIgA2m2. (D) Same as in (B)
but for pentameric sIgA2m2.RESEARCH | RESEARCH ARTICLE