N-terminal Transmembrane-Helix Epitope Tag for X-ray Crystallography and Electron Microscopy of Small Membrane Proteins

•Antibody complexation aids structural studies of small membrane proteins.•The 17-residue MPER peptide from HIV gp41 can be grafted to diverse membrane proteins.•The MPER epitope tag is a contiguous, structured extension of the N-terminal helix.•Antibodies that bind to the MPER epitope can serve as crystallization chaperones.•MPER-binding antibodies can serve as electron microscopy fiducial markers.