EXTRACELLULARLY OCCURRING HISTONE H1 MEDIATES THE BINDING OF THYROGLOBULIN TO THE CELL-SURFACE OF MOUSE MACROPHAGES

Thyroglobulin is the major secretory protein of thyroid epithelial cel ls. Part of thyroglobulin reaches the circulation of vertebrates by tr anscytosis across the epithelial wall of thyroid follicles. Clearance of thyroglobulin from the circulation occurs within the liver via inte rnalization of thyroglobulin by macrophages. Here we have analyzed the interaction of thyroglobulin with the cell surface of J774 macrophage s with the aim to identify the possible thyroglobulin-binding sites on macrophages. Binding of thyroglobulin to J774 cells was saturated at similar to 100 nM thyroglobulin with a K-d Of 50 nM, and it was compet ed by the ligand itself. Preincubation of J774 cells with thyroglobuli n resulted in downregulation of thyroglobulin-binding sites, indicatin g internalization of thyroglobulin and its binding proteins. By affini ty chromatography, two proteins from J774 cells were identified as thy roglobulin-binding proteins with an apparent molecular mass of similar to 33 kD Unexpectedly, both proteins were identified as histone H1 by protein sequencing. The occurrence of histone H1 at the plasma membra ne was further proven by biotinylation or immunolabeling of J774 cells . The in vitro interaction between histone HI and thyroglobulin was an alyzed by surface plasmon resonance that revealed a K-d at 46 nM. In s itu, histone H1 was colocalized to FITC-Tg-containing endocytic compar tments of Kupffer cells, i.e., liver macrophages. We conclude that his tone H1 is detectable at the cell surface of macrophages where it serv es as a thyroglobulin-binding protein and mediates thyroglobulin endoc ytosis.