K. Brix et al., EXTRACELLULARLY OCCURRING HISTONE H1 MEDIATES THE BINDING OF THYROGLOBULIN TO THE CELL-SURFACE OF MOUSE MACROPHAGES, The Journal of clinical investigation, 102(2), 1998, pp. 283-293
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.