Two-State Conformations in the Hyaluronan-Binding Domain Regulate CD44 Adhesiveness under Flow Condition

SummaryThe hyaluronan (HA) receptor CD44 mediates cell adhesion in leukocyte trafficking and tumor metastasis. Our previous nuclear magnetic resonance (NMR) studies revealed that the CD44 hyaluronan-binding domain (HABD) alters its conformation upon HA binding, from the ordered (O) to the partially disordered (PD) conformation. Here, we demonstrate that the HABD undergoes an equilibrium between the O and PD conformations, in either the presence or absence of HA, which explains the seemingly contradictory X-ray and NMR structures of the HA-bound HABD. An HABD mutant that exclusively adopts the PD conformation displayed a higher HA affinity than the wild-type. Rolling of the cells expressing the mutant CD44 was less efficient than those expressing the wild-type, due to the decreased tether frequency and the slow cellular off rate. Considering that the mutant CD44, devoid of the low-affinity state, exhibited impaired rolling, we conclude that the coexistence of the high- and low-affinity states of the HABD is essential for the CD44-mediated rolling.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (135 K)Download as PowerPoint slideHighlights► CD44 HABD exists in two-state equilibrium either in the ligand-free or bound states ► Ligand-binding shifts the equilibrium from the O form to the PD form ► O and PD form represents low- and high-affinity states for ligand, respectively ► Two-state equilibrium is crucial for CD44-mediated cell rolling under flow condition