Effect of cytoplasmic domain mutations on the agonist-stimulated ligand binding activity of the platelet integrin alpha IIb beta 3

Function of the platelet integrin alpha IIb beta 3 is regulated by agonist-generated signals interacting with its cytoplasmic tails, When alpha IIb beta 3 is expressed in Epstein-Barr virus-transformed B lymphocytes, stimulation of the cells with phorbol 12-myristate 13-acetate results in alpha IIb beta 3-mediated lymphocyte adherence to immobilized fibrinogen, as well as soluble fibrinogen binding to alpha IIb beta 3, indicating that agonists increase the affinity of alpha IIb beta 3 for fibrinogen in these cells, To address the contribution of the alpha IIb and beta 3 cytoplasmic tails to this process, we mutated each tail and expressed the mutants in B lymphocytes. Truncation of the alpha IIb tail did not impair unstimulated or stimulated lymphocyte adherence to fibrinogen, regardless whether the truncation was proximal or distal to the conserved GFFKR sequence. However, deleting GFFKR or replacing it with alanines markedly reduced alpha IIb beta 3 expression due to impaired intracellular assembly of alpha IIb beta 3 heterodimers, probably due to a mutation-induced change in the conformation of alpha IIb. Introducing beta 3 mutations known to impair alpha IIb beta 3 function in platelets into the cytoplasmic tail of beta 3 in lymphocytes also impaired alpha IIb beta 3 function in these cells, These studies demonstrate that the cytoplasmic tail of alpha IIb is not required for alpha IIb beta 3 function in lymphocytes, although the presence of GFFKR in the alpha IIb, tail is required for alpha IIb to interact with beta 3. Additionally, they indicate that signals interacting with the beta 3 cytoplasmic tail are responsible for the ability of agonists to stimulate alpha IIb beta 3 function.