Development of a structural model for the cytoplasmic domain of an integrin

The cytoplasmic tails of integrin heterodimers play central roles in controlling the activation states of integrins and in transmitting intracellular signals. Despite their short length, no structure of any integrin cytoplasmic domain has been determined. Therefore, molecular models for the cytoplasmic domain of alpha(IIb)beta(3), the major platelet integrin, were generated, including models for the individual cytoplasmic tails, the binary alpha(IIb)-calcium complex, and the ternary alpha(IIb)-beta(3)-calcium complex. Structural analysis of circular dichroism spectra were compiled with data obtained from short homologous sequences within crystallized proteins, and with secondary structural predictions to develop starting models for each subunit. These models were subjected to a series of energy minimization and molecular dynamic simulations to generate final models. alpha(IIb) was predicted to be ordered at its N-terminus and its C-terminus could accommodate a cation in a multicoordinated complex. The structure of beta(3) was dominated by a beta-turn at its NPXY motif (beta(3)744-747). In docking of alpha(IIb) to different sites within beta(3), the conformation of the beta(3) juxta-transmembrane (beta(3)716-721) was greatly altered. This region was confirmed to be a conformational 'hot-spot' by circular dichroism. The conformational flexibility of this juxta-transmembrane region, which is highly conserved amongst integrins, is ideally located to regulate signaling.