Exposure of cells to intense light with the photoactivatable reagent, N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine), present in the external medium results in irreversible inhibition of chloride or sulfate exchange. This irreversible inhibition seems to result from covalent reaction with the same sites to which NAP-taurine binds reversibly in the dark. As shown in the preceding paper, high chloride concentrations decrease the reversible inhibition by NAP-taurine in the dark, in a manner suggesting that NAP-taurine and chloride compete for the modifier site of the anion transport system. In a similar fashion, high chloride concentrations in the medium during exposure to light cause a decrease in both the irreversible binding of NAP-taurine to the membrane and the inhibition of chloride exchange. Most of the chloride-sensitive irreversibly bound NAP-taurine is found in the 95,000 dalton polypeptide known as band 3 and, after pronase treatment of intact cells, in the 65,000 dalton fragment of this protein produced by proteolytic cleavage. After chymotrypsin treatment of ghosts, the NAP-taurine is localized in the 17,000 dalton transmembrane portion of this fragment. Although the possible involvement of minor labeled proteins cannot be rigorously excluded, the modifier site labeled by external NAP-taurine appears, therefore, to be located in the same portion of the 95,000 dalton polypeptide as is the transport site.

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