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

The platelet GPIIb-GPIIIa heterodimer (integrin alphaIIbbeta3) binds fibrinogen with high affinity in response to activation by agonists, leading to platelet aggregation and formation of a hemostatic plug. The 326GRV motif in GPIIb is highly conserved in the alpha-subunit of other integrins, suggesting that it might play an important functional role. Moreover, Arg327-->His substitution in GPIIb has been associated with defective platelet surface expression of GPIIb-IIIa and thrombasthenic phenotype. This work aimed at elucidating whether the absence of Arg327 or its 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 into Chinese hamster ovary cells inherently expressing GPIIIa permitted surface exposure of GPIIb-IIIa complexes, whereas [Glu327]GPIIb did not. These observations indicate that it is not the loss of [Arg327]GPIIb but the presence of His327 or a negatively charged residue like Glu at position 327 of GPIIb that prevents the surface exposure of GPIIb-IIIa heterodimers. In contrast, changing Gln344, the homologue to Arg327 in the alpha-subunit of the vitronectin receptor, to His did not prevent the surface expression of alphav-GPIIIa complexes. Thus the conformational constraint imposed by His327 seems to be rather specific for the heterodimerization and/or processing of GPIIb-IIIa complexes.