Structural basis of transcobalamin recognition by human CD320 receptor

Area of Science:

• Biochemistry
• Structural Biology
• Cell Biology

Background:

• Cellular uptake of vitamin B12 (cobalamin) is mediated by the cell surface receptor CD320, a member of the LDLR family.
• CD320 captures transcobalamin (TC), the vitamin B12 carrier in plasma, to facilitate cellular entry.

Purpose of the Study:

• To elucidate the structural basis of the interaction between human holo-TC and the extracellular domain of CD320.
• To understand the molecular mechanisms underlying TC-CD320 binding affinity and ligand release.

Main Methods:

• X-ray crystallography was used to determine the structure of the holo-TC/CD320 complex.
• Solid-phase binding assays and thermostability analysis were employed to quantify binding affinity and complex stability.
• Structures of complexes involving a disease-associated CD320 mutant and an LDLR-A2 domain were also determined.

Main Results:

• The crystal structure of the human holo-TC/CD320 complex reveals the detailed interaction interface.
• High affinity of CD320 for TC and selective binding over haptocorrin are explained by the observed interaction chemistry.
• Stable TC-CD320 complexes were observed for a disease mutant and an LDLR-A2 domain, with minor structural differences.
• TC-CD320 binding affinity significantly decreases at low pH, mimicking endocytic ligand release.

Conclusions:

• The study provides a structural understanding of vitamin B12 receptor-ligand interactions.
• The findings rationalize the high affinity and specificity of CD320 for TC.
• The pH-dependent binding explains the proposed mechanism of ligand release during cellular uptake via endocytosis.