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Figure 5.
Fig. 5. Models for the CHIR-AB1/FcY interaction. (a) Sequence
alignment of the extracellular domains of CHIR-AB1 and CHIR AB2
with the D1 domains of CHIR-A2, CHIR-B2, and CHIR-AB3 (GenBank
accession numbers AJ745094, AJ745095, AJ745093, AJ639837, and
AJ879909, respectively). Residues at the dimer interface of
CHIR-AB1 are indicated by asterisks above the sequence. (b) The
ribbon diagram of a CHIR-AB1 dimer with residues that differ
from CHIR-AB2 is highlighted as red (dimer interface residues)
or blue (all others) sticks. The highlighted amino acids are
labeled. (c) Potential models for binding between CHIR-AB1 and
IgY. The CHIR-AB1 ectodomain is in blue, the ITIM in the
cytoplasmic tail is represented by a rectangle, and the cell
membrane is shown as a dotted black line. IgY is shown with a
yellow and orange Fc region and gray Fab. Left: A dimer of
CHIR-AB1 is bound asymmetrically to the lower hinge region
between the C[H]2 and the C[H]3 domains of FcY, analogous to the
binding of FcγRs and FcεRI to Fcs. Middle: CHIR-AB1 monomers
bind to the C[H]3–C[H]4 interdomain interface to create a
symmetrical 2:1 complex, analogous to the binding of FcαRI to
Fc. Right: The 2-fold symmetry axis of a CHIR-AB1 dimer aligns
with the 2-fold symmetry axis of FcY to form a symmetrical 2:1
complex in which each CHIR-AB1 monomer binds to the bottom of a
FcY C[H]4 domain.
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