Molecular basis for immune complex recognition: A comparison of Fc-receptor structures

Once antigen is opsonised by IgG it is removed from the circulation by Fc gamma -receptor expressing cells. Fc gamma -receptors are type I transmembrane molecules that carry extracellular parts consisting of two or three immunoglobulin domains. Previously solved structures of Fc-receptors reveal that the N-terminal two Ig-like domains are arranged in a steep angle forming a heart-shaped structure. The crystal structure of the Fc gamma RIII/ hlgG1-Fc-fragment demonstrated that the Fc-fragment is recognised through loops of the C-terminal receptor domain of the Fc gamma RIII. As the overall structure of the FcRs and their Ig-ligands are very similar we modelled the Ig complexes with Fc gamma RI, Fc gamma RII and Fc epsilon RI alpha based on the Fc gamma RIII/hIgG1-Fc-fragment structure. The obtained models are consistent with the observed biochemical data and may explain the observed specificity and affinities. (C) 2001 Academic Press.