Colicin Ia undergoes a transition from a soluble to a transmembrane state, forming an ion channel to effect its bactericidal activity. The X-ray crystal structure of soluble colicin Ia at an effective resolution of 4 Å reveals that the molecule is highly α-helical and has an unusually elongated ‘Y’-shape. The stalk and two arms of the ‘Y’ form three discrete structural domains which most likely correspond to the three functional regions identified for the channel-forming colicins. The channel-forming region of colicin Ia can be located to the larger of the two arms, the insertion domain, by its structural similarity to the ten α-helix motif found for the ion channel-forming fragments of colicins A and E1. The domain arrangement found in this structure provides novel insights into the mechanism of membrane insertion of colicin Ia.
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Ghosh, P., Mel, S. & Stroud, R. The domain structure of the ion channel-forming protein colicin Ia. Nat Struct Mol Biol 1, 597–604 (1994). https://doi.org/10.1038/nsb0994-597