Subunit Structure of Islets-Activating Protein (IAP), a New Protein Isolated from the Culture Media of Bordetella pertussis

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Abstract
The subunit structure was studied of islets-activating protein (IAP), a new protein recently isolated from the culture media of Bordetella pertussis and possessing a unique action, i.e., potentiating insulin secretory responses of animals, IAP dissociated into three subunits, F-l, F-2, and F-3, when incubated in 8 M urea. Three subunits isolated by chromatography on CM-Sepharose and DEAE-Sepharose columns showed different molecular weights (F-l: 44,000, F-2: 20,000, F-3: 11,000) and different isoelectric points, but similar amino acid compositions. The F-l subunit consisted of two polypeptide chains linked by S-S bonding(s), while the F-2 and F-3 subunits were single-chain peptides. These subunits, none of which was biologically active alone, associated upon incubation for 2 h at 37°C and regained biological activities after association only when the F-3 subunit was present in the association product. Thus, the F-3 subunit was essential, and the F-l and F-2 subunits were permissive, for the development of IAP activity in animals.