Human granulocyte/pollen-binding protein. Recognition and identification as transferrin

Area of Science:

• Immunology
• Biochemistry
• Protein Science

Background:

• Human serum contains various proteins with diverse biological functions.
• Granulocyte binding to allergens like timothy grass pollen is a key event in allergic responses.
• The specific proteins mediating this interaction have not been fully elucidated.

Purpose of the Study:

• To identify and characterize the heat-stable protein in normal human serum that promotes granulocyte binding to timothy grass pollen.
• To investigate the biochemical properties and identity of this granulocyte/pollen-binding protein (GPBP).

Main Methods:

• Purification of GPBP using sequential chromatography techniques: gel filtration, anion exchange, and affinity chromatography.
• Characterization of GPBP's molecular mass and isoelectric point.
• Identification of GPBP using immunoelectrophoresis and SDS-PAGE.
• Confirmation of GPBP identity using monospecific antisera and comparison with purified transferrin.

Main Results:

• A heat-stable protein, GPBP, was isolated and found to promote granulocyte binding to timothy grass pollen.
• GPBP was identified as transferrin, a beta-1-protein with a molecular mass of ~77,000 D and an isoelectric point of 5.5–6.1.
• GPBP activity was neutralized by antisera against transferrin, confirming their identity.
• Observed heterogeneity in GPBP was minimized when saturated with ferric ions, suggesting a role for iron binding.

Conclusions:

• Granulocyte/pollen binding is a novel, unrecognized property of transferrin.
• This function of transferrin appears independent of its canonical iron transport role.
• Transferrin may possess a physiological function in the clearance of specific types of organic matter.