changes, leading to the formation of a very
different triangular shape (Fig. 4C). D4-D5 ro-
tates 120° en bloc to become co-linear with D2-
D3, and the newly formed D3-D4 interface buries
a large surface area (1486 Å^2 ). D1 is rotated 84°
and packs onto D2-D3. This large conforma-
tional change is consistent with a previous
double electron-electron resonance (DEER)
spectroscopy study ( 22 ). In particular, the dis-
tance between the Caatoms of D1-Thr^67 and
D5-Gln^491 is 78 Å in this new conformation,
in good agreement with DEER measurements
showing that ligand binding induces a sepa-
ration of these two residues beyond 70 Å. The
three CDRs function as a central hub to me-
diate a network of interactions with Fcm1, Fcm5,
and the J-chain (Fig. 4, A, D, and E, and fig.
S3,ItoN).InCDR1,pIgR/SCresidueVal^29 in-
teracts with J-chain residue Ala^132 in the
J-chain C-terminal hairpin (Fig. 4D). pIgR/SC
residue Asn^30 forms a hydrogen bond with
Liet al.,Science 367 , 1014–1017 (2020) 28 February 2020 3of4
AB
CDR2
CDR3
E53P
Y55P
R451Fcμ1A
R514Fcμ1A
E468Fcμ1B
N97P
L466Fcμ1B Y576 R99P
Fcμ5B
T574Fcμ5A
L101P
Y576Fcμ5A
P544Fcμ1A
E468Fcμ1B
Y576Fcμ5B
V29P
A132J
N30P
H32P
Y134J
R34P
CDR1
R105J
DE
F
Strep
pull-down
Input
SC-His
Strep-IgM-Fc
J chain-His
WT
+
+
+
+
+
+
WTV29N/R31SR99N/L101T
anti His (SC)
anti Strep
anti His (J)
anti His (SC)
anti Strep
anti His (J)
C
D5
D2
D3
D4
CDR1 CDR2
CDR3
T67
Q491
28 Å
D1
S T
D5
D2
D3
D4
CDR2 CDR1
CDR3
T67
Q491
78 Å
D1
A
Fcμ 1
Fcμ 5
J
CDR1
CDR2
CDR3
SC
D1
Fig. 4. Conformational change of pIgR/SC and its interaction with the
Fcm-J complex.(A) The three CDRs (magenta) in pIgR/SC-D1 (gold)
interact with Fcm1(blue),Fcm5 (cyan), and the J-chain (red). pIgR/SC is
shown as a surface representation. Its D2 to D5 domains are shown in
white. (B) The structure of apo pIgR/SC (PDB ID 5D4K). D1 is shown in gold,
with the three CDR loops highlighted in magenta. The Caatoms of D1-Thr^67
and D5-Gln^491 are shown as spheres, and the distance between them is
indicated. (C)ThestructureofpIgR/SCintheFcm-J-SC complex.
(D) Detailed view of the interactions at the D1-CDR1 region. Polar
interactions are indicated by dashed lines. (E) Detailed view of the
interactions at the CDR2 and CDR3 region. (F)SCmutantsdisplay
reduced interactions with Fcm-J.
ABA
β 1 C68J
β 2
β 4 β^3
C14J
C575Fcμ5B
Fcμ1A
Fcμ5B
N-wing
C-wing
Fcμ5A
Fcμ4B
Y134J
P130J
A127JT126J
V113J
L115J
P114J
F358
P544
V547
V124J
L359
F485
P494
M489
V537
C575Fcμ1A
C575Fcμ5A
C575Fcμ4B
Fig. 3. Structure of the J-chain and its interactions with Fcm.(A) The J-chain has a two-winged
structure and interacts with the tailpieces of the Fcmpentamer. (B) The C-terminal hairpin of the J-chain
interacts with Fcm1A.
RESEARCH | REPORT