Aromatic and cation-π interactions enhance helix-helix association in a membrane environment

The cation−π interaction is an electrostatic attraction between a positive charge and the conjugated π electrons of an aromatic ring. These interactions are well documented in soluble proteins and can be both structurally and functionally important. Catalyzed by observations in our laboratory that an Ala- and Ile-rich two-helix transmembrane segment tended to form SDS-resistant dimers upon the incorporation of suitably located Trp residues, here we have constructed a library of related constructs to study systematically the impact of aromatic−aromatic and cation−π interactions on tertiary structure formation within an Escherichia coli membrane. Using the TOXCAT oligomerization assay with the hydrophobic segment AIAIAIIAZAXAIIAIAIAI, where Z = A, W, Y, or F and X = A, H, R, or K in all possible combinations of cation and/or aromatic pairings, to assess the TM−TM dependent expression of the chloramphenicol acetyltransferase reporter gene, we find that cation−π interactions, particularly between Lys and Trp,...