The Concave Face of Decorin Mediates Reversible Dimerization and Collagen Binding*

Abstract

Background: In the crystal structure of decorin, the concave faces of two monomers interact to form a tight dimer.

Results: The decorin dimer in solution is in equilibrium with stable monomers, and mutations on the concave face abolish collagen binding.

Conclusion: Decorin binds collagen as a monomer.

Significance: These findings help resolve the controversy about the functional oligomeric state of decorin.

Abstract

Decorin, the prototypical small leucine-rich proteoglycan, binds to collagen and thereby regulates collagen assembly into fibrils. The crystal structure of the decorin core protein revealed a tight dimer formed by the association of two monomers via their concave faces (Scott, P. G., McEwan, P. A., Dodd, C. M., Bergmann, E. M., Bishop, P. N., and Bella, J. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 15633–15638). Whether decorin binds collagen as a dimer has been controversial. Using analytical ultracentrifugation, we determined a dissociation constant of 1.37 ± 0.30 μ m for the mouse decorin dimer. Dimerization could be abolished by engineering glycosylation sites into the dimer interface; other interface mutants remained dimeric. The monomeric mutants were as stable as wild-type decorin in thermal unfolding experiments. Mutations on the concave face of decorin abolished collagen binding regardless of whether the mutant proteins retained the ability to dimerize or not. We conclude that the concave face of decorin mediates collagen binding and that the dimer therefore must dissociate to bind collagen.