N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate (NAP-taurine) as a photoaffinity probe for identifying membrane components containing the modifier site of the human red blood cell anion exchange system.

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Abstract
Exposure of cells to intense light with the photoactivatable reagent, NAP-taurine, present in the external medium resulted in irreversible inhibition of Cl- or sulfate exchange. This irreversible inhibition seemed to result from covalent reaction with the same sites to which NAP-taurine binds reversibly in the dark. High Cl- concentrations decreased the reversible inhibition by NAP-taurine in the dark, in a manner suggesting that NAP-taurine and Cl- compete for the modifier site of the anion transport system. In a similar fashion, high Cl- concentrations in the medium during exposure to light caused a decrease in the irreversible binding of NAP-taurine to the membrane and the inhibition of Cl- exchange. Most of the Cl-sensitive irreversibly bound NAP-taurine was 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 was 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 appeared located in the same portion of the 95,000 dalton polypeptide as was the transport site.