Primary and secondary structure of PF4 in relation to HIT neoepitopes.

(Top) A 3-dimensional representation of the PF4 tetramer is shown, indicating the 2 neoepitope sites identified by Li et al.⁶ The “ring of positive charge” formed by the lysine residues in the C-terminus of PF4 (light blue) and other lysine and arginine residues (dark blue) is also shown. (Bottom) The linear sequence of the 70–amino acid polypeptide of a single PF4 molecule is shown. Four such polypeptides combine to form the PF4 tetramer. PF4 is classified as a member of the C-X-C subfamily of chemokines because of its cysteine₁₀-leucine₁₁-cysteine₁₂ sequence.

Abbreviations: PF4, platelet factor 4.

Adapted with permission from Li et al.⁶