Colicin A immunity protein interacts with the hydrophobic helical hairpin of the colicin A channel domain in the Escherichia coli inner membrane

The colicin A pore-forming domain (pfCoIA) was fused to a bacterial signal peptide (sp-pfCoIA). This was inserted into the Escherichia coli inner memb rane in functional form and could be coimmunoprecipitated with epitope-tagg ed immunity protein (EpCai). We constructed a series of fusion proteins in which various numbers of sp-pfCoIA alpha -helices were fused to alkaline ph osphatase (A-P). We showed that a fusion protein made up of the hydrophobic a-helices 8 and 9 of sp-pfCoIA fused to AP was specifically coimmunoprecip itated with EpCai produced in the same cells. This is the first biochemical evidence that Cai recognizes and interacts with the colicin A hydrophobic helical hairpin.