The E-helix is a central core in a conserved helical bundle involved in nucleotide binding and transmembrane domain intercalation in the ABC transporter superfamily

Abstract

ABC transporter proteins are involved in active transport, both in prokaryotes and eukaryotes. Sequence analysis of nucleotide binding domains (NBDs) of ABC proteins from all taxa revealed a well-conserved new motif having the signature: xT/ShxE/DNhxF, located between Q-loop and ABC signature sequence. A recent structure of an ABC transporter, ABCG5/G8 highlighted the motif as an essential structural determinant of inter-domain crosstalk and termed it as E-helix. We carried out an extensive computational analysis to unravel important structural entities alongside E-helix which plausibly play role in the interlocking mechanism of NBD with TMD. We identified E-helix to be a central structural moiety which interacts with three helices and an intracellular loop that leads to the transmembrane domain. Considering its wide occurrence, we examined the importance of this motif in one representative multidrug ABC transporter of Candida albicans, Cdr1p. The motif residues were replaced by alanines both individually as well as in combinations. The GFP-tagged versions of mutant proteins were overexpressed in Saccharomyces cerevisiae. Overall, our mutational data suggested that this motif plays a role in the maintenance of proper structural fold and/or inter-domain contacts in Cdr1p. We, thus, unveil an essential structural motif in ABC superfamily transporters.

Keywords: ABC superfamily; E-helix; Nucleotide-binding domains.