Design and Chemical Synthesis of a Neoprotein Structural Model for the Cytoplasmic Domain of a Multisubunit Cell-Surface Receptor: Integrin α_IIbβ₃ (Platelet GPIIb-IIIa)

Integrins are a class of heterodimeric cell adhesion receptors involved in cell migration, cell anchorage, and cell-cell interactions. The cytoplasmic domains of integrins are of key importance in these activities. We have designed and chemically synthesized a 126 amino acid model protein (MP-1) containing both cytoplasmic tails of the platelet-derived integrin αIIbβ₃ covalently linked via a helical coiled coil. The coiled-coil tertiary structure was incorporated to mimic the membrane-spanning domain of the integrin and to act as a topological constraint fixing the two cytoplasmic tails in a parallel arrangement. This molecule, which contains two C-termini, was constructed by chemical dovetailing. The bromoacetylated and cysteinyl peptide synthons were unambiguously ligated through the formation of a thioether linkage. Ultraviolet circular dichroism (CD) spectroscopy has been performed on MP-1 and related compounds, confirming that a helical coiled coil is present within the MP-1 molecule. Significantly, the helicity apparently extends beyond the predicted amphiphilic region of MP-1. Fluorescence measurements suggest that a defined tertiary structure has formed by the association of the two cytoplasmic domains. We conclude that this is a practical design strategy for the study of the cytoplasmic domain of multisubunit cell-surface receptors.