Unchanged Transmembrane Segment Kink and Topology Despite Variation in Membrane-Mimetic Environment

Abstract

Membrane mimetics such as micelles, bicelles or bilayers provide an essential tool for studying membrane proteins and their fragments. Following the “divide and conquer” strategy, we have determined the structure of the N-terminal tail and first transmembrane segment of the human apelin receptor (residues 1-55, AR55). This was solved using solution-state nuclear magnetic resonance (NMR) spectroscopy in sodium dodecylsuphate (SDS), dodecylphosphocholine (DPC) and 1-palmitoyl-2-hydroxy-sn-glycero-3-phospho-(1′-rac-glycerol) (LPPG) micelles as well as in a solution of 50% H2O and 50% hexafluoroisopropanol (HFIP). This high-resolution structure of AR55 consists of a helix-kink-helix motif in the micelle-spanning transmembrane region in all of the membrane mimetics studied. The N-terminal tail of AR55 has a similar structure in all three micelle conditions and has a much higher helical content in HFIP. Dynamics along the polypeptide backbone from 15N nuclear spin relaxation measurements of AR55 are correlated to solvent accessibility determined by paramagnetic relaxation enhancement, allowing definitive comparison of the manner of embedding of the protein. We show that AR55 adopts a highly similar conformation and topology in SDS, DPC and LPPG micelles despite the dramatic differences between these micelles in both headgroup and tailgroup. The transmembrane domain conformation of AR55 in HFIP is largely similar, including at the kink, despite the induction of helical character in the solution exposed N-terminal tail.