a, ALKAL binds to the TNF-α like region of ALK. b, The primary ligand-interacting residues on ALK involve the C-terminal loop of the TNF-α like region. The distal segment of the loop, including glutamates E974 and E978 is detailed. An additional glutamate, E859, outside the TNF-α like region interacts with the ALKAL2-AD. The positively charged arginine-containing surface of ALKAL faces the negatively charged glutamate cluster of ALK. c, Interactions involving the non-polar initial segment of ALK’s C-terminal loop (Y966-L970) are detailed. d, Statistically significant protection seen in HDX-MS studies mapped onto the crystal structure of human ALK GRD. Regions that show statistically significant protection when bound by ALKAL are colored blue. e, Percent exchange plots for the C-terminal loop peptides 964–970 and 967–977 with or without ALKAL2-AD as a function of exposure time. Data represent mean ± SD of two biologically independent measurements of which three technical repeats were carried out.