Mannose-specific lectins bind alpha-2-macroglobulin and an unknown protein from human plasma

Area of Science:

• Biochemistry
• Proteomics
• Immunology

Background:

• Alpha-2-macroglobulin (A2M) is a large plasma proteinase inhibitor.
• Mannose-specific lectins, like GNA from Galanthus nivalis, bind to A2M.
• An unknown protein was copurified with A2M during lectin-based isolation.

Purpose of the Study:

• To characterize the unknown protein copurified with A2M.
• To investigate the binding specificities of various lectins with A2M and its family members.
• To determine if the unknown protein is related to A2M or other known plasma proteins.

Main Methods:

• Affinity purification using immobilized mannose-specific lectins (GNA, NPA, RSA).
• Analysis of protein properties including molecular mass, isoelectric point, and N-terminal sequencing.
• Immunological assays using antisera against the unknown protein.
• Peptide sequencing after enzymatic digestion and CNBr fragmentation.

Main Results:

• NPA lectin showed higher binding capacity for A2M isolation than GNA.
• The unknown protein shares a mannose carbohydrate structure with A2M.
• The unknown protein has a subunit size of 185 kDa and native mass of 350 kDa, forming a noncovalent homodimer.
• The protein is present in all human plasma samples tested and shows no homology to known protein sequences.
• It is not a proteinase inhibitor and is distinct from A2M family members.

Conclusions:

• An unknown human plasma protein, distinct from A2M and other known proteins, has been identified.
• This novel protein possesses a unique mannose carbohydrate structure recognized by specific lectins.
• The protein's structure and lack of homology suggest it represents a new class of plasma protein.