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TraM is essential for F plasmid‐mediated bacterial conjugation, where it binds to the plasmid DNA near the origin of transfer, and recognizes a component of the transmembrane DNA transfer complex, TraD. Here we report the 1.40 Å crystal structure of the TraM core tetramer (TraM^58–127). TraM^58–127 is a compact eight‐helical bundle, in which the N‐terminal helices from each protomer interact to form a central, parallel four‐stranded coiled‐coil, whereas each C‐terminal helix packs in an antiparallel arrangement around the outside of the structure. Four protonated glutamic acid residues (Glu88) are packed in a hydrogen‐bonded arrangement within the central four‐helix bundle. Mutational and biophysical analyses indicate that this protonated state is in equilibrium with a deprotonated tetrameric form characterized by a lower helical content at physiological pH and temperature. Comparison of TraM to its Glu88 …