THE TIP OF THE HYDROPHOBIC HAIRPIN OF COLICIN-U IS DISPENSABLE FOR COLICIN-U ACTIVITY BUT IS IMPORTANT FOR INTERACTION WITH THE IMMUNITY PROTEIN

The hydrophobic C terminus of pore-forming colicins associates with an d inserts into the cytoplasmic membrane and is the target of the respe ctive immunity protein. The hydrophobic region of colicin U of Shigell a boydii was mutated to identify determinants responsible for recognit ion of colicin U by the colicin U immunity protein. Deletion of the ti p of the hydrophobic hairpin of colicin U resulted in a fully active c olicin that was no longer inactivated by the colicin U immunity protei n. Replacement of eight amino acids at the tip of the colicin U hairpi n by the corresponding amino acids of the related colicin B resulted i n colicin U(575-582ColB), which was inactivated by the colicin U immun ity protein to 10% of the level of inactivation of the wild-type colic in U. The colicin B immunity protein inactivated colicin U(575-582ColB ) to the same degree. These results indicate that the tip of the hydro phobic hairpin of colicin U and of colicin B mainly determines the int eraction with the corresponding immunity proteins and is not required for colicin activity. Comparison of these results with published data suggests that interhelical loops and not membrane helices of pore-form ing colicins mainly interact with the cognate immunity proteins and th at the loops are located in different regions of the A-type and El-typ e colicins, The colicin U immunity protein forms four transmembrane se gments in the cytoplasmic membrane, and the N and C termini face the c ytoplasm.