Afamin is a serum glycoprotein that stabilizes and transports hydrophobic signaling protein Wnt3a, but the structural basis of this function has remained unclear. Here, we combined high-speed atomic force microscopy and atomistic molecular modeling to investigate this complex in solution. Our analysis revealed that afamin exhibits the motion of two globular domains opening and closing through the hinge regions. An Afamin/Wnt3a complex adopts two interconvertible conformations: a symmetric conformation, where Wnt3a positions at the center between the two globular domains of afamin, and an asymmetric conformation, where Wnt3a positions on the N-terminal-side domain of the afamin. Binding to Wnt3a reduced the intrinsic flexibility of afamin. Additionally, cell-based binding experiments demonstrated that stable Wnt3a association requires the intact architecture of afamin containing an undisturbed hydrophobic pocket region. Together, these findings suggest that the Afamin/Wnt3a complex adopts its dynamic states, with the lipid moiety of Wnt3a embedded in the hydrophobic pocket of afamin.
Ichida et al. (Wed,) studied this question.