S-[2-(4-Azidosalicylamido)ethylthio]-2-thiopyridine:  Radioiodinatable, Cleavable, Photoactivatible Cross-Linking Agent

S-[2-(4-Azidosalicylamido)ethylthio]-2-thiopyridine (AET) contains a 2-thiopyridyl moiety, which permits cysteine-specific incorporation into protein through a cleavable disulfide bond, and a 4-azidosalicylamido moiety, which permits radioiodination and photoactivatible cross-linking. In contrast to the related compound S-[2-[N-[4-(4-azidosalicylamido)butyl]carbomoyl]ethylthio]-2 -thiopyridine [APDP; Zecherle, G., Oleinikov, A., and Traut, R. (1992) Biochemistry 31, 9526], AET contains a relatively short linker arm between the 2-thiopyridyl moiety and the 4-azidosalicylamido moiety. In a previous paper, it was shown that AET could be used in site-specific protein-protein photocross-linking to identify nearest-neighbor protein domains within a multiprotein complex [Chen, Y., Ebright, Y., and Ebright, R. (1994) Science 265, 90]. In this paper, the synthesis, radioiodination, and incorporation into protein of AET are described.