INVESTIGATION OF THE STRUCTURAL COMPONENTS GOVERNING THE POLARITY-DEPENDENT REFOLDING OF A CD4-BINDING PEPTIDE FROM GP120

The conformational switch at the principle CD4-binding domain of gp120 from HIV1 exhibits a highly cooperative folding transition from beta- sheet to helix triggered within a very narrow range of solvent polarit y. The physical basis of this folding behaviour is of interest because it is unusual and because it is closely connected with biological fun ction, i.e. binding to the CD4 receptor. Previous work revealed two pr imary structural elements, an N-terminal LPCR tetrad and a tryptophan residue eight residues C-terminal to this, that were essential for the helical and for the beta-sheet conformation, respectively Attempts to construct synthetic ''switch'' domains using the characteristics so f ar identified produce peptides undergoing the transition at much highe r polarity and involving fewer residues than the natural domain, in es sence a lower stability of the beta-fold to apolar conditions. Introdu ction of a tryptophan residue reduced at the C-(2)-C-(3) linkage demon strates clearly that the aromatic system of the tryptophan residue is central to beta-sheet stabilization. Residues with side-chains that mi ght participate in electrostatic or aromatic interactions with the pi- electron system of Trp were sequentially altered to alanine. The resul ts indicate that the ''switch'' properties of the CD4-binding domain a rise from a poised tension between multiple interactions with the Trp aromatic ring stabilizing the beta-structure and the tendency of the L PCR tetrad to act as a template for a helical fold. Under polar condit ions the former dominate. Lowering the polarity alters this both by we akening the aromatic interactions and by simultaneously increasing the helical propensities of the isoleucine and valine side-chains. Trypto phan seems uniquely suited to act as polarity-sensitive conformational sensor.