DOMINANT ROLE OF LOCAL DIPOLES IN STABILIZING UNCOMPENSATED CHARGES ON A SULFATE SEQUESTERED IN A PERIPLASMIC ACTIVE-TRANSPORT PROTEIN

Electrostatic interactions are among the key factors determining the s tructure and function of proteins. Here we report experimental results that illuminate the functional importance of local dipoles to these i nteractions. The refined 1.7-angstrom X-ray structure of the liganded form of the sulfate-binding protein, a primary sulfate active transpor t receptor of Salmonella typhimurium, shows that the sulfate dianion i s completely buried and bound by hydrogen bonds (mostly main-chain pep tide NH groups) and van der Waals forces. The sulfate is also closely linked, via one of these peptide units, to a His residue. It is also a djacent to the N-termini of three alpha-helices, of which the two shor test have their C-termini 'capped' by Arg residues. Site-directed muta genesis of the recombinant Escherichia coli sulfate receptor had no ef fect on sulfate-binding activity when an Asn residue was substituted f or the positively charged His and the two Arg (changed singly and toge ther) residues. These results, combined with other observations, furth er solidify the idea that stabilization of uncompensated charges in a protein is a highly localized process that involves a collection of lo cal dipoles, including those of peptide units confined to the first tu rns of helices. The contribution of helix macrodipoles appears insigni ficant.