Im4

From Proteopedia

Im4 is an immunity protein that is produced by E. coli at the same time as Colicin E4 to protect itself from the Colicin's cytotoxic 16s rRNase activity. It binds to the C domain and inhibits its enzymatic activity, to protect the colicinogenic cell from death. It is later released from this domain when the colicin binds BtuB, the receptor on the target cell.