The Hydrophilic Domain of HSulf Endosulfatases: An Intrinsically Disordered Region Governing Enzyme Functions and Therapeutic Potential.

Human extracellular endosulfatases HSulf-1 and HSulf-2 catalyze the selective 6-O-desulfation of heparan sulfate (HS), critically shaping the sulfation code that governs glycosaminoglycan (GAG)-mediated signaling. A unique feature of these enzymes is their hydrophilic domain (HD), an intrinsically disordered, non-conserved segment absent from all other human sulfatases. Despite lacking homology to known protein domains, the HD has emerged as a key regulatory module for substrate recognition, enzyme localization, and processive activity along HS chains. In this review, we dissect the structural and functional roles of the HD, with emphasis on its dynamic interaction with HS motifs and potential modulation of protein-GAG complexes. We also explore how its intrinsically disordered nature may confer conformational flexibility advantageous for navigating the complex landscape of extracellular glycans. Given the implication of HSulfs in diverse physiological and pathological contexts, including cancer, the HD presents a promising therapeutic target for the selective inhibition of endosulfatase activity. We discuss the challenges and perspectives in targeting intrinsically disordered regions (IDRs) in GAG-binding proteins, and highlight how the HD of HSulfs provides a paradigmatic example of non-canonical domains orchestrating fine-tuned GAG editing in the extracellular matrix.