High-resolution crystal structures of alternate forms of the human CD44 hyaluronan-binding domain reveal a site for protein interaction

Two new crystal structures of the extracellular hyaluronan-binding domain of human CD44 are described at high resolution. A hexagonal crystal form at 1.60Å resolution and a monoclinic form at 1.08Å resolution both have two molecules in the asymmetric unit arranged about a similar noncrystallographic twofold axis of symmetry. These structures are compared with those previously reported at 2.20Å resolution to show that the fold is quite resistant to structural deformation in different crystal environments. Unexpectedly, a short peptide is found in the monoclinic crystals at a site remote from the known hyaluronan-binding groove. The peptide with a valine at the carboxy-terminus must have co-purified from the bacterial expression host and binds on the opposite side of the domain from the known hyaluronan-binding groove. This opportunistic binding may identify a site of interaction used as CD44 assembles with other proteins to accomplish effective signaling regarding changes to the extracellular environment.