Role of the alpha -subunit 326GRV sequence in the surface expression of fibrinogen and vitronectin receptors

The platelet GPIIb-GPIIIa heterodimer (integrin_IIb3)binds fibrinogen with high affinity in response to activation byagonists, leading to platelet aggregation and formation of a hemostatic plug. The ³²⁶GRV motif in GPIIb ishighly conserved in the -subunit of other integrins, suggesting thatit might play an important functional role. Moreover,Arg³²⁷His substitution inGPIIb has been associated with defective platelet surface expression ofGPIIb-IIIa and thrombasthenic phenotype. This work aimed at elucidatingwhether the absence of Arg³²⁷ orits substitution by His was responsible for the impaired surface expression of GPIIb-IIIa complexes. Transfection of cDNA encoding [Ala³²⁷]GPIIb,[Gln³²⁷]GPIIb, or[Phe³²⁷]GPIIb intoChinese hamster ovary cells inherently expressing GPIIIa permittedsurface exposure of GPIIb-IIIa complexes, whereas [Glu³²⁷]GPIIb did not.These observations indicate that it is not the loss of[Arg³²⁷]GPIIb but thepresence of His³²⁷ or a negativelycharged residue like Glu at position 327 of GPIIb that prevents thesurface exposure of GPIIb-IIIa heterodimers. In contrast, changingGln³⁴⁴, the homologue toArg³²⁷ in the -subunit of thevitronectin receptor, to His did not prevent the surface expression of_v-GPIIIacomplexes. Thus the conformational constraint imposed byHis³²⁷ seems to be rather specificfor the heterodimerization and/or processing of GPIIb-IIIacomplexes.