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Structural basis of PSGL‐1 binding to ERM proteins

P‐selectin glycoprotein ligand‐1 (PSGL‐1), an adhesion molecule with O‐glycosylated extracellular sialomucins, is involved in leukocyte inflammatory responses. On activation, ezrin–radixin–moesin (ERM) proteins mediate the redistribution of PSGL‐1 on polarized cell surfaces to facilitate binding to target molecules. ERM proteins recognize a short binding motif, Motif‐1, conserved in cytoplasmic tails of adhesion molecules, whereas PSGL‐1 lacks Motif‐1 residues important for binding to ERM proteins. The crystal structure of the complex between the radixin FERM domain and a PSGL‐1 juxtamembrane peptide reveals that the peptide binds the groove of FERM subdomain C by forming a β‐strand associated with strand β5C, followed by a loop flipped out towards the solvent. The Motif‐1 310 helix present in the FERM–ICAM‐2 complex is absent in PSGL‐1 given the absence of a critical Motif‐1 alanine residue, and PSGL‐1 reduces its contact area with subdomain C. Non‐conserved positions are occupied by large residues Met9 and His8, which stabilize peptide conformation and enhance groove binding. Non‐conserved residues play an important role in compensating for loss of binding energy resulting from the absence of conserved residues important for binding.