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Three-dimensional structures of complexes of the SH2 domain of the v-src oncogene product with two phosphotyrosyl peptides have been determined by X-ray crystallography at resolutions of 1.5 and 2.0 A, respectively. A central antiparallel beta-sheet in the structure is flanked by two alpha-helices, with peptide binding mediated by the sheet, intervening loops and one of the helices. The specific recognition of phosphotyrosine involves amino-aromatic interactions between lysine and arginine side chains and the ring system in addition to hydrogen-bonding interactions with the phosphate.

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A novel series of receptors for guanine has been prepared and is shown, by 1H n.m.r., to bind the nucleotide base via both hydrogen bonding and aromatic stacking interactions.Recent discoveries of the role played by guanine nucleotide binding proteins (G-proteins) in the function of cell-surface receptors1 have focused much attention2 on the biochemical recognition and complexation of guanine. One possible strategy involves coupling hydrogen bonding to the periphery of the purine base with a stacking interaction perpendicular to its plane.3 This is shown by ribonuclease TI which binds