Structure of a C-type mannose-binding protein complexed with an oligosaccharide

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• C-type (calcium-dependent) animal lectins are key mediators of cell-surface carbohydrate recognition.
• Mannose-binding proteins (MBPs) are a significant class of C-type lectins involved in innate immunity and cellular processes.

Purpose of the Study:

• To elucidate the structural basis of carbohydrate recognition by C-type lectins.
• To understand the role of calcium ions in stabilizing the interaction between mannose-binding proteins and carbohydrate ligands.

Main Methods:

• X-ray crystallography was employed to determine the high-resolution (1.7 Å) crystal structure of the carbohydrate-recognition domain of rat mannose-binding protein.
• The protein was co-crystallized with an oligomannose asparaginyl-oligosaccharide ligand.

Main Results:

• The crystal structure revealed that calcium ions (Ca2+) form direct coordination bonds with the carbohydrate ligand.
• A network of coordination and hydrogen bonds was identified, stabilizing the ternary complex of protein, Ca2+, and sugar.
• The structure demonstrated how oligosaccharide branches can crosslink neighboring carbohydrate-recognition domains, enabling visualization of multivalent binding.

Conclusions:

• The findings provide detailed insights into the molecular mechanisms underlying carbohydrate specificity in C-type lectins.
• Calcium ions are critical for mediating and stabilizing lectin-carbohydrate interactions.
• The study visualizes multivalent binding, highlighting the importance of protein domain arrangement in ligand recognition.