Crystal structure of human chondroadherin: solving a difficult molecular-replacement problem using de novo models

Area of Science:

• Structural Biology
• Biochemistry
• Biophysics

Background:

• Chondroadherin (CHAD) is a crucial cartilage matrix protein involved in chondrocyte adhesion.
• CHAD interacts with collagen, proteoglycans, and integrins, with its integrin-binding site located in an uncharacterized C-terminal region.
• Therapeutic peptides derived from CHAD show promise for osteoporosis treatment.

Purpose of the Study:

• To determine the three-dimensional structure of human chondroadherin (hCHAD).
• To elucidate the structure of the C-terminal integrin-binding site.
• To provide insights for improving therapeutic peptides targeting CHAD.

Main Methods:

• Structure solution using de novo model generation with Rosetta and phasing via AMPLE.
• Overcoming challenges with traditional molecular replacement due to low sequence identity and crystallographic symmetry.
• Crystal structure determination and docking simulations.

Main Results:

• The crystal structure of hCHAD revealed an ordered C-terminal domain with the LRRCT fold.
• The integrin-binding motif (WLEAK) is situated within a canonical alpha-helix.
• Docking simulations and the crystal structure support hCHAD dimerization similar to other matrix LRR proteins.

Conclusions:

• The determined hCHAD structure provides the first atomic-level view of its C-terminal domain and integrin-binding site.
• The findings offer a structural basis for understanding CHAD's interactions and for rational design of enhanced therapeutic peptides.
• The study demonstrates the utility of de novo modeling for solving complex protein structures.