Activity of protein MalE (maltose-binding protein) fused to cytoplasmic and periplasmic regions of an Escherichia coli inner membrane protein

We analysed the properties of mature MBP (maltose-binding protein or MaIE protein) fused to an integral cytoplasmic membrane protein of Escherichia coli. Fusion of MaIE to the first MaIG periplasmic loop enabled a strain defective in the maIE gene to utilize maltose. In contrast, fusion of MaIE to a cytoplasmic loop did not complement the maIE Delta 444 deletion. We obtained results highly correlated with those obtained by using alkaline phosphatase as a reporter for the topology of MaIG. We discuss the possibility of genetically determining the topology of cytoplasmic membrane proteins by a method based on engineered fusions to MBP.