Makoto Tamura, Katsumi Nogimori, Satoshi Murai, Motoyuki Yajima, Kiyoshi Ito, ToshiakiKatada, Michio Ui,* and Shinichi Ishii abstract: The subunit structure of islet-activatingprotein (IAP), pertussis toxin, has been analyzed to study a possibility that this protein is one of the AB toxins [Gill, D. M.(1978) in Bacterial Toxins and Cell Membranes (Jeljaszewicz, J., & Wadstrom, T., Eds.) pp 291-332, Academic Press, New York], Heating IAP with 1% sodium dodecyl sulfate caused its dissociationinto five dissimilar subunits named Sl (with a molecular weight of 28000), S-2 (23 000), S-3 (22000), S-4 (11 700), and S-5 (9300), as revealed by polyacrylamide gel electrophoresis; their molar ratio in the native IAP was 1: 1: 1: 2: 1. The molecularweight of IAP estimated by equi-librium ultracentrifugation was 117 000 which was not at variance with the value obtained by summing up molecular weights of the constituent subunits. The preparative separation of these IAP subunits was next undertaken; exposure of IAP to 5 M ice-cold urea for 4 days followed by column chromatography with carboxymethyl-Sepharose caused sharp sepa-ration of Sl and S-5, leaving the other subunits as two dimers. These dimers were then dissociated into their constituent subunits, ie, S-2 and S-4 for one dimer and S-3 and S-4 for