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'''Platelet membrane glycoproteins''' are surface [[glycoproteins]] found on [[platelets]] ([[thrombocytes]]) which play a key role in [[hemostasis]]. When the [[blood vessel]] wall is damaged, '''platelet membrane glycoproteins''' interact with the [[extracellular matrix]].

==Receptors involved in platelet adhesion to collagen==
Membrane glycoproteins [[GPIa/IIa]], [[GPVI]] and probably [[CD36|GPIV]] as well, function as [[receptor (biochemistry)|receptors]] engaged in platelet [[adhesion]] to [[collagen]]. The leading role in the elimination of high-stress injury is taken by the [[GPIb-IX-V]] complex.

==Interactions of the platelet surface glycoproteins==
The binding of [[von Willebrand factor]] (vWF) results in conformational changes within the GPIb-V-IX complex. In consequence, this complex activates GPIIb / IIIa membrane glycoproteins allowing them to bind [[fibrinogen]]. Fibrinogen molecules then interconnect the platelets serving as the basis for platelet aggregation. In the absence of fibrinogen, the platelets are joined by vWF due to its ability to bind the activated GPIIb / IIIa complex.

==Membrane glycoproteins==

===Glycoprotein Ib-IX-V complex  ([[GPIb-IX-V]])===
This transmembrane glycoprotein complex is composed of four subunits: [[GP1BA|GPIbα]], [[GP1BB|GPIbβ]],  [[Glycoprotein V|GPV]] and [[Glycoprotein IX|GPIX]]. Each of them has a variable number of [[leucine-rich repeats]]. GPIbα and GPIbβ are linked by [[disulfide bridges]], while the GPV and GPIX associate non-covalently with the complex. GPIbα subunit bears the binding site for [[von Willebrand factor]] (vWF), [[thrombin|α-thrombin]], leukocyte [[integrin]] [[Macrophage-1 antigen|αMβ2]] and [[P-selectin]]. The binding between GPIbα and vWF mediates the capture of platelets to the injured vascular wall. The deficiency in glycoprotein Ib-IX-V complex synthesis leads to [[Bernard-Soulier syndrome]].<ref name="LEPAGE">LEPAGE, A., M. LEBOEUF, J.P. CAZENAVE, C. SALLE, F. LANZA a G. UZAN. The αIIbβ3 integrin and GPIb-V-IX complex identify distinct stages in the maturation of CD34+cord blood cells to megakaryocytes. Blood journal of the American Society of Hematology [online]. [cit. 2012-06-12]. {{ISSN|1528-0020}}.http://bloodjournal.hematologylibrary.org/content/96/13/4169.full.pdf
</ref>

===Glycoprotein VI ([[GPVI]])===
Glycoprotein VI is one of the [[immunoglobulin superfamily]] [[transmembrane protein|type I transmembrane glycoproteins]]. It is an important collagen receptor involved in collagen-induced platelet activation and adhesion. It plays a key role in their procoagulant activity and subsequent [[thrombin]] and [[fibrin]] formation. Its procoagulant function may contribute to arterial or venous [[thrombosis]]. The [[Fc receptor|FCR]] pathway of [[GPVI]] activation involves [[γ chain]] (GPVI transmembrane domain associates with γ chain FCR), [[Src kinase]] [[FYN]] / [[LYN]], and [[LAT1|LAT]] adaptor protein, all participating in [[phospholipase C]] activation.<ref name="Jandrot-Perrus">Jandrot-Perrus M, Busfield S, Lagrue AH, Xiong X, Debili N, Chickering T, Le Couedic JP, Goodearl A, Dussault B, Fraser C, Vainchenker W, Villeval JL (September 2000). "Cloning, characterization, and functional studies of human and mouse glycoprotein VI: a platelet-specific collagen receptor from the immunoglobulin superfamily". Blood 96 (5): 1798–807. {{PMID|10961879}}.</ref>

===Glycoprotein Ia / IIa complex  ([[GPIa / IIa]] = integrin [[CD49b|α<sub>2</sub>]][[CD29|β<sub>1</sub>]])===
This is a receptor for [[collagen type I]] and [[collagen type IV|IV]]. It consists of two subunits (α<sub>2</sub> and β<sub>1</sub>). The α<sub>2</sub> subunit includes a domain homologous to von Willebrand factor domain binding to collagen. The β<sub>1</sub> subunit has four cysteine-rich regions and a structure similar to other β-integrins. The interaction with collagen leads to stabilization of the platelets. The surface expression of this complex shows high variability, particularly in relation to the polymorphism of GPIa subunit gene. Different opinions exist on the importance of C - T point mutation at position 807 which is believed to be associated with the risk of [[myocardial infarction]] or [[ischemic stroke]].<ref>ESTAVILLO, RITCHIE, DIACOVO a CRUZ. Functional Analysis of a Recombinant Glycoprotein Ia/IIa (Integrin α2β1) I Domain That Inhibits Platelet Adhesion to Collagen and Endothelial Matrix under Flow Conditions. The Journal of Biological Chemistry,. s. -. DOI: 10.1074/jbc.274.50.35921. Available online: http://www.jbc.org/content/274/50/35921.long</ref>

===Glycoprotein IIb / IIIa complex  ([[Glycoprotein IIb/IIIa|GPIIb / IIIa]] = integrin [[ITGA2B|α<sub>IIb</sub>]][[CD61|β<sub>3</sub>]])===
This complex interacts with fibrinogen and thus plays an important role in platelet aggregation and adhesion to [[endothelial]] surfaces. Activation of this complex initiates the platelet aggregation and the formation of primary platelet plug, a [[fibrin]] [[clot]]. The IIb / IIIa complex is a major platelet membrane component. There are as many as 50 000 copies. The α<sub>IIb</sub> (GPIIb) is composed of two subunits linked by [[disulfide bridges]]. The β<sub>3</sub> (GPIIIa) forms a single polypeptide chain. These subunits form Ca<sup>2+</sup> - dependent complex on the surface of platelet membrane in a 1:1 ratio.

'''Fibrinogen sites recognized by glycoprotein IIb / IIIa complex:'''
* dodecapeptide located in the C-terminal of the fibrinogen γ chain (the most important)
* RGD sequence of the α chain → the [[Arginine]]-[[Glycine]]-[[Aspartate]] amino acid sequence

This complex also binds vWF, [[fibronectin]] and [[vitronectin]]. In the resting state the contact between the two protein subunits (necessary for the complex activation)is prevented by aggregin, which disables their contact necessary for the complex activation. The complex can be activated by [[Adenosine diphosphate|ADP]]. When bound to ADP, conformational changes occur within the aggregin molecule and consequently, it dissociates from the two subunits. Furthermore, the complex can be activated by thrombin. Thrombin binding to its receptor activates [[protein kinase C]] and increases the level of [[inositol triphosphate]]. Consequently, there is a release of calcium ions that activate [[calpain]]. Calpain cleaves aggregin, and thus allows for joining of the two subunits. Deficiency in the IIb / IIIa complex is described as [[Glanzmann's thrombasthenia]]. Patients completely lack the ability to aggregate platelets.<ref name="SHATTIL">SHATTIL, S. J., J. A. HOXIE, M. CUNNINGHAM a BRASS. Changes in the platelet membrane glycoprotein IIb.IIIa complex during platelet activation. The Journal of Biological Chemistry. September 15, 1985. {{ISSN|0021-9258}}. Available online: http://www.jbc.org/content/260/20/11107.short</ref>

===GPV / IIIa ([[GPV / IIa]] = integrin [[ITGA5|α<sub>5</sub>]][[CD61|β<sub>1</sub>]])===
This is a [[heterodimer]].Its α<sub>5</sub> subunit is 36% identical to the GPIIb subunit. This complex is located mainly on endothelial cells but also on smooth muscle cells, [[macrophages]] and [[platelets]]. Its main function is in the adhesion of cells to the extracellular matrix components.

==References==

{{Reflist}}

==External links==
* Calvete JJ (1995). "On the structure and function of platelet integrin alpha IIb beta 3, the fibrinogen receptor". Proc. Soc. Exp. Biol. Med. 208 (4): 346–60. {{PMID|7535429}}.
* GARNER, CAMPBELL a METCALFE. Glycoprotein V: the predominant target antigen in gold-induced autoimmune thrombocytopenia. Blood journal of the American Society of Hematology. 2002. {{ISSN|1528-0020}}. DOI: 10.1182/blood.V100.1.344.Available online: http://bloodjournal.hematologylibrary.org/content/100/1/344.full.pdf
* Dumin, J A; Dickeson S K, Stricker T P, Bhattacharyya-Pakrasi M, Roby J D, Santoro S A, Parks W C (Aug. 2001). "Pro-collagenase-1 (matrix metalloproteinase-1) binds the alpha(2)beta(1) integrin upon release from keratinocytes migrating on type I collagen". J. Biol. Chem. (United States) 276 (31): 29368–74. DOI:10.1074/jbc.M104179200. {{ISSN|0021-9258}}. {{PMID|11359786}}.
* http://www.reference.md/files/D019/mD019038.html
* [http://cancerweb.ncl.ac.uk/cgi-bin/omd?platelet+membrane+glycoproteins Online Medical Dictionary, Platelet membrane glycoproteins]
* {{MeshName|Platelet+membrane+glycoproteins}}
* http://www.ojrd.com/content/1/1/46/figure/F1?highres=y
* http://circ.ahajournals.org/content/99/1/e1/F4.large.jpg
{{Coagulation}}

[[Category:Coagulation system]]
[[Category:Glycoproteins]]

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