J-chain residue Arg^105 , which in turn packs onto
Tyr^576 of Fcm5B. pIgR/SC residue His^32 packs
against J-chain residue Tyr^134 , which interacts
with Pro^544 of Fcm1A as described above. pIgR/
SC residue Arg^34 forms a salt bridge with
Glu^468 of Fcm1B. In CDR2 and CDR3, pIgR/
SC residue Glu^53 interacts with both Arg^451 and
Arg^514 of Fcm1A (Fig. 4E). It has been reported
that rabbit and rodent pIgR can transport IgA
but not IgM ( 23 , 24 ). Rabbit pIgR lacks a res-
idue entirely at this position, whereas rodents
feature an Asn ( 4 , 21 ). Arg^451 and Arg^514 are
also not conserved in mouse IgM (fig. S5A).
pIgR/SC residue Tyr^55 packs against Glu^468 of
Fcm1B. pIgR/SC residue Asn^97 interacts with
the main-chain carbonyl group of Leu^466 of
Fcm1B. pIgR/SC residue Arg^99 is sandwiched
between Tyr^576 of Fcm5B and Thr^574 of Fcm5A.
pIgR/SC residue Leu^101 interacts with Tyr^576
of Fcm5A and Tyr^576 of Fcm5B. Two SC mutants,
V29N/R31S and R99N/L101T, which are de-
signed to introduce bulky N-linked glycans in
the CDR1 and CDR3 regions, respectively, dis-
play greatly reduced interactions with the Fcm-
J complex (Fig. 4F), confirming the functional
relevance of the molecular interactions de-
scribed above.
Two other IgM receptors exist in mammals
in addition to pIgR: FcamRandFcmR/Toso/
Faim3 ( 25 ). They each contain a domain that is
homologous to the D1 domain of pIgR. Like
pIgR, FcamR binds both IgA and IgM. Its D1-
like domain also shows high sequence similarity
to pIgR-D1 (fig. S5B), and residues correspond-
ing to pIgR/SC residues Val^29 , Asn^30 , His^32 ,
Arg^34 , Tyr^55 , and Leu^101 that are involved in
binding to the Fcm-J complex in pIgR are all
present. Thus, FcamRmayinteractwithIgMin
a manner similar to pIgR. FcmR/Toso/Faim3,
on the other hand, binds only to IgM. Further-
more, it can interact with both pentameric
and hexameric IgM with similar affinities ( 26 ).
This is in contrast to pIgR, which selectively
binds to the IgM pentamer that contains the
J-chain. Indeed, the D1-like domain of FcmR/
Toso/Faim3 is more divergent than pIgR-D1
and lacks most of the critical Fcm-J–interacting
residues (fig. S5C). Thus, FcmR likely binds IgM
in a different fashion.
Our high-resolution cryo-EM structure of
the Fcm-J-SC complex provides a framework
for further understanding the functions of
IgM, and also sheds light on the interac-
tion between IgM and other receptors. As
a result of the stronger binding of IgM to
its targets and its more potent activity to
induce complement-dependent cytotoxicity,
IgM can be potentially exploited for ther-
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ACKNOWLEDGMENTS
We thank the Core Facilities at the School of Life Sciences, Peking
University for help with negative-staining EM; the Cryo-EM Platform
of Peking University for help with data collection; the High-
Performance Computing Platform of Peking University for help
with computation; the National Center for Protein Sciences at
Peking University for assistance with Amersham Imager; and G. Li for
help with cDNA library.Funding:Supported by the National Key
Research and Development Program of China (2017YFA0505200
to J.X.; 2016YFC0906000 to J.X. and X.-D.S.; 2019YFA0508904 to
N.G.), the National Science Foundation of China (31570735,
31822014 to J.X.; 31725007, 31630087 to N.G.; 21727806 to
X.-D.S.; 31922036 to N.L.), the Qidong-SLS Innovation Fund to
J.X. and N.G.; and the Clinical Medicine Plus X Project of Peking
University to J.X. and Y.T.Author contributions:Y.L. performed
protein purification; Y.L. and G.W. prepared the cryo-EM sample
and collected data; G.W. and N.L. processed the cryo-EM data;
J.X. and N.G. built the structural model; Y.L. performed the pull-
down experiments; J.X. wrote the manuscript, with inputs from
all authors.Competing interests:The authors declare no
competing financial interests.Data and materials availability:
The cryo-EM map and atomic coordinates of the Fcm-J-SC complex
have been deposited in the EMDB and PDB with accession codes
EMD-0782 and 6KXS, respectively. All constructs used for protein
expression in this study are available upon request.SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/367/6481/1014/suppl/DC1
Materials and Methods
Figs. S1 to S5
Table S1
References ( 27 – 36 )18 September 2019; accepted 24 January 2020
Published online 6 February 2020
10.1126/science.aaz5425Liet al.,Science 367 , 1014–1017 (2020) 28 February 2020 4of4
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