Revealing the mechanism of protein-lipid interactions for a putative membrane curvature sensor in plant endoplasmic reticulum

Membrane curvature sensing via helical protein domains, such as those identified in Amphiphysin and ArfGAP1, have been linked to a diverse range of cellular processes. However, these regions can vary significantly between different protein families and thus remain challenging to identify from sequence alone. Greater insight into the protein-lipid interactions that drive this behavior could lead to production of therapeutics that specifically target highly curved membranes. Here we demonstrate the curvature-dependence of membrane binding for an amphipathic helix (APH) in a plant reticulon, namely RTNLB13 from A. thaliana. We utilize solution-state nuclear magnetic resonance spectroscopy to establish the exact location of the APH and map the residues involved in protein-membrane interactions at atomic resolution. We find that the hydrophobic residues making up the membrane binding site are conserved throughout all A. thaliana reticulons. Our results also provide mechanistic insight that leads us to propose that membrane binding by this APH may act as a feedback element, only forming when ER tubules reach a critical size and adding stabilization to these structures without disrupting the bilayer. A shallow hydrophobic binding interface appears to be a feature shared more broadly across helical curvature sensors and would automatically restrict the penetration depth of these structures into the membrane. We also suggest this APH is highly tuned to the composition of the membrane in which it resides, and that this property may be universal in curvature sensors thus rationalizing the variety of mechanisms reported for these functional elements.