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Figure 5.
Fig. 5. Structural illustration of integrin activation by
talin. (A) A model for membrane-mediated change of cytoplasmic
face during integrin activation. Agonist stimulation induces a
conformational change in talin that exposes its head domain
(Talin-H). Talin-H binds to the  [3] tail at both the
NPLY-containing region and the membrane-proximal helix. The
binding to the membrane-proximal region displaces the  [IIb]
tail from its complex with the [3] tail, leading to an
unclasping, and the binding in the NPLY region releases a
membrane-anchoring constraint on 3, which further
facilitates the unclasping movement along the membrane surface.
Notice the shifted membrane interface for both membrane-proximal
helices before and after unclasping (green bars), which suggests
a "fanning-out" unclasping process because the transmembrane
domains may also undergo separation or open-scissor motion. The
unclasping initiates the opening of the integrin C-terminal
stalks, which is necessary for the rearrangement of the
extracellular headpiece for high-affinity ligand binding. (B)
HSQC spectra of the 15N-labeled 3 tail in the absence
(black) and presence (red) of unlabeled talin F2-F3 at 35°C.
Residues with significant chemical shift changes were labeled,
which primarily involve membrane-proximal T720-D723 and
C-terminal A735-A750, containing the N744-Y747 turn. (C) Surface
plasmon resonance data. One hundred nanomol of talin-H (1-429)
was passed over CM5 sensor chips coated with [3] (716-762) (red), a
[3]
membrane-proximal mutant (H722A/D723A, black), or a [3]
single mutant (F730A, green), with association and dissociation
phases of 360 sec. The former mutant had diminished binding to
talin, but the latter has about the same as the binding capacity
to talin, indicating that H722D723 is critical for talin
binding. D723A/R724A mutations also had the same effect as
H722A/D723A (data not shown). Talin-H made no detectable
interaction with [IIb](989-1008) when
this peptide was coupled to a CM5 chip.
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