Background: Support for the role of transmembrane and membrane-proximal domains of alpha IIb beta 3 integrin in the maintenance of receptor low affinity comes from mutational studies showing that activating mutations can induce constitutive bi-directional transmembrane signaling.
Design and methods: We report the functional characterization of a mutant alpha IIb beta 3 integrin carrying the Leu718Pro mutation in the membrane-proximal region of the beta 3 cytoplasmic domain, identified in heterozygosis in a patient with a severe bleeding phenotype and defective platelet aggregation and adhesion.
Results: Transiently transfected cells expressed similar levels of normal and mutant alpha IIb beta 3, but surface expression of mutant alpha v beta 3 was reduced due to its retention in intracellular compartments. Cells stably expressing mutant alpha IIb beta 3 showed constitutive binding to soluble multivalent ligands as well as spontaneous fibrinogen-dependent aggregation, but their response to DTT was markedly reduced. Fibrinogen-adherent cells exhibited a peculiar spreading phenotype with long protrusions. Immunofluorescence analysis revealed the formation of alpha IIb beta 3 clusters underneath the entire cell body and the presence of atypical high-density patches of clustered alpha IIb beta 3 containing encircled areas devoid of integrin that showed decreased affinity for the fluorescent lipid analog DiIC(16) and were disrupted in cholesterol-depleted cells.
Conclusions: These findings are consistent with an important role of the membrane-proximal region of beta 3 in modulating alpha IIb beta 3 clustering and lateral redistribution of membrane lipids. Since the beta 3 mutant was associated with a thrombasthenic phenotype in a patient carrying one normal beta 3 allele, these results support a dominant role of clustering in regulating integrin alpha IIb beta 3 functions in vivo.