Page 1887 - Williams Hematology ( PDFDrive )

1862 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1863

protein with four characteristic divalent cation-binding sites domain explains why there is a “long range” disulfide bond extending
(see Fig. 112–11). The mature protein contains 1008 amino acids 43,854 from C13 to C435; thus, even though the βA domain makes contact
with one transmembrane domain; during processing, it is cleaved into a with the integrin α propeller (via Arg261 and other residues that inter-
IIb
heavy chain and a light chain connected by a disulfide bond. The β sub- act with two rings of hydrophobic residues in the integrin α “cage”),
IIb
unit, β , contains 762 amino acids and is rich in cysteine residues, with a it is not the aminoterminus of the molecule. The PSI domain contains
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A1
characteristic cysteine-rich region near its transmembrane domain. 43,855 Leu33, which defines the Pl (HPA-1a) specificity, as opposed to the
A2
The integrin α and β cytoplasmic tails consist of 20 and 47 amino alloantigen Pl (HPA-1b), which is produced by a Pro33 polymor-
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IIb
acids, respectively. The genes coding for α and β are very close to phism (Chap. 137). The integrin β leg is composed of four integrin
IIb
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each other on chromosome 17 at q21.32, but are not so close as to share EGF domains that are rich in disulfide bonds. In the crystal structure,
common regulatory domains. 856,857 Both proteins are synthesized in this region interacts with the integrin α stalk region and the globu-
IIb
megakaryocytes and join to form a calcium-dependent, noncovalent lar head in the bent, unactivated receptor, but these interactions are
858
complex in the rough endoplasmic reticulum. Calnexin probably less prominent in the three-dimensional reconstruction of the inactive
serves as a chaperone for integrin α , but it is unclear which chaper- receptor not in the activated receptor. 250,827,848 Mutations in the integrin
859
IIb
one(s) are involved in integrin β folding and/or integrin α β complex EGF domains, including cysteine residues, can activate the receptor as
IIb 3
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formation. The integrin α β complex subsequently undergoes further can the binding of monoclonal antibodies. 879–882 The importance of the
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processing in the Golgi apparatus, where the carbohydrate structures normal disulfide bond pairings in integrin β is further supported by
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undergo maturation and the pro-GPIIb molecule is cleaved into its data demonstrating that certain reducing agents can cause activation of
heavy and light chains by furin or a similar enzyme. 860,861 Approximately integrin α β , fibrinogen binding, and platelet aggregation, 883,884 and an
IIb 3
15 percent of the mass of both integrins α and β are composed of car- enzyme capable of catalyzing the exchange of thiol groups and disulfide
IIb
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bohydrate. The mature integrin α β complex is then transported to in proteins (PDI) has been identified on the surface of platelets and in
862
IIb 3
the plasma membrane or the membranes of α granules or dense bodies. platelet releasates. 883,885–887 Thiol-disulfide exchange in integrins α β
IIb 3
If integrins α and β do not form a proper complex, either because of a and α β is implicated as a contributor to clot retraction. Moreover,
888
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V 3
IIb
structural abnormality in either subunit or the failure to synthesize one regions in integrin β itself have the same consensus sequence (CGXC)
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of the subunits, the subunit(s) that are synthesized are rapidly degraded present in PDI that is thought to mediate the catalysis. One model
889
and so are not expressed on the membrane surface (Chap. 121). Deg- suggests that integrin α β can achieve a low level of activation without
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radation of integrin α appears to involve retro-translocation from the alterations in disulfide bonds, but that maximal activation requires PDI
IIb
endoplasmic reticulum into the cytoplasm, ubiquitination, and prote- or similar activity along with a source of thiols such as plasma glutathi-
olysis by the megakaryocyte proteasome. 859 one or a membrane NAD(P)H oxidoreductase system. Inhibition of
883
Both integrins α and β are composed of a series of domains PDI and other enzymes that mediate thiol-disulfide exchange (ERp57,
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IIb
(see Fig. 112–11). The aminoterminal region of integrin α contains a ERp5) reduces platelet thrombus formation. 890,891 It is still unclear, how-
IIb
seven-blade β-propeller domain, and each blade is composed of four β ever, whether disulfide bond alterations contribute to activation in vivo
strands connected by loops. The propeller interacts with the βA (I-like) under physiologic or pathologic conditions.
domain of integrin β , forming the globular head region observed in Transmembrane domain structures of integrin α and integrin
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IIb
electron micrographs. The four calcium ions bound by the propeller β have been proposed based on NMR and structural modeling stud-
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domain interact with β hairpin loops in blades four to seven that extend ies. 871,873,874,892–896 Because the integrin α transmembrane helix is shorter
IIb
away from the interface with integrin β . In addition, there is a unique than the integrin β helix, they traverse the membrane at an angle of
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integrin α cap subdomain made up of four loops from blades one to approximately 25 degrees. The association of the integrin α and inte-
IIb
IIb
three that are unique to α and contribute to its ligand binding speci- grin β ectodomains near the site of entry into the membrane results
IIb
3
ficity. The remainder of the extracellular components of integrin α are in the transmembrane helices being directly juxtaposed in the region
IIb
250
made up of a thigh, genu (knee-like), and two calf domains, much of the membrane closest to the ectodomain (outer membrane clasp).
like the structure of the related integrin α subunit. 841,844 The cytoplasmic Near the cytoplasmic end of the membrane the helices are held together
V
domain of integrin α interacts with the cytoplasmic domain of integ- by an inner membrane clasp composed of the integrin α residues
IIb
IIb
rin β and the interaction is important in controlling activation of the immediately after the end of the helix (GFFKR), with the membrane
3
holoreceptor. 863–866 The cytoplasmic domain of integrin α has a GFFKR reimmersion of F992 and F993 filling the gap and interacting with inte-
IIb
sequence near the membrane that is thought to control inside-out acti- grin β W715 and I719, with integrin α R995 creating a salt bridge
3
IIb
vation of the integrin receptors because mutations or deletions in this with integrin β 723 and perhaps residue 726. 897,898 Of note, these regions
3
region result in the receptor adopting a conformation with high affinity are conserved in many other integrins receptors and so the basic mech-
for fibrinogen. 867–871 A number of studies using mutagenesis and nuclear anism may be common to many of the receptors.
magnetic resonance (NMR) identified different structures for the trans- Inside-out signaling is accomplished by the talin F3 domain bind-
membrane and cytoplasmic domains, and differences in the relative roles ing to the integrin β cytoplasmic domain, which is proposed to disrupt
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of heterodimeric and homodimeric associations. 864,872–875 Disrupting the the inner membrane clasp. 34,244,245,863,865,866,869,870,872,876,892,899,900 This may be
conformation of this region also results in a constitutively high-affinity facilitated by migfilin displacing filamin from the integrin β cytoplas-
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901
receptor, 876,877 which has led to the conclusion that inside-out activation mic domain as the latter interaction may prevent talin binding. Talin
of integrin α β requires separation of the transmembrane and cyto- binding results in dissociation of the transmembrane helices and reor-
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plasmic domains, but it remains possible that more subtle changes in the ganization of the cytoplasmic region of integrin β into a more extended
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cytoplasmic and transmembrane domains may be sufficient. 848 helix. Integrin α β ectodomain chain separation, headpiece exten-
IIb 3
The integrin β subunit domains are not linearly arranged because sion, and integrin β swing out then follow, either spontaneously or as
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the first domain (PSI [plexins, semaphorins, and integrins]) was sub- a result of the traction force generated by the cytoskeleton on integrin
149
jected to the insertion of a hybrid domain, which itself was subjected β through talin. Outside-in signaling is presumed to be initiated by
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to the insertion of a βA (I-like) domain; the latter domain is homol- loss of ectodomain interactions between the membrane-proximal regions
ogous to the VWF A domain and integrin I domains, both of which of integrins α and β , perhaps as a result of ligand binding producing
IIb
3
bind ligands (see Fig. 112–11). 827,878 The double insertion in the PSI even greater integrin β swing out, resulting in disruption of the outer
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Kaushansky_chapter 112_p1829-1914.indd 1862 17/09/15 3:29 pm