Design of amphiphilic protein maquettes: Controlling assembly, membrane insertion, and cofactor interactions

We have designed polypeptides combining selected lipophilic (LP) and hydrophilic (HP) sequences that assemble into amphiphilic (AP) alpha-helical bundles to reproduce key structure characteristics and functional elements of natural membrane proteins. The principal AP maquette (API) developed here joins 14 residues of a heme binding sequence from a structured diheme-four-alpha-helical bundle (HP1), with 24 residues of a membrane-spanning LP domain from the natural four-alpha-helical M2 channel of the influenza virus, through a flexible linking sequence (GGNG) to make a 42 amino acid peptide. The individual API helices (without connecting loops) assemble in detergent into four-a-helical bundles as observed by analytical ultracentrifugation. The helices are oriented parallel as indicated by interactions typical of adjacent hemes. AP1 orients vectorially at nonpolar-polar interfaces and readily incorporates into phospholipid vesicles with >97% efficiency, although most probably without vectorial bias. Mono- and diheme-AP1 in membranes enhance functional elements well established in related HP analogues. These include strong redox charge coupling of heme with interior glutamates and internal electric field effects eliciting a remarkable 160 mV splitting of the redox potentials of adjacent hemes that leads to differential heme binding affinities. The AP maquette variants, AP2 and AP3, removed heme-ligating histidines from the HP domain and included heme-ligating histidines in LP domains by selecting the b(H) heme binding sequence from the membrane-spanning d-helix of respiratory cytochrome bc(1). These represent the first examples of AP maquettes with heme and bacteriochlorophyll binding sites located within the LP domains.