Leukocyte common antigens (CD45) are large receptors that are abundantly expressed at the surface of all leukocytes. These receptors are type I membrane glycoproteins possessing two large C-terminal intracellular domains with protein tyrosine phosphatase activity. While the role of these enzyme domains in leukocyte signaling is well documented, the role of the N-terminal extracellular portion of CD45, composed of sequences formed by alternatively spliced exons, the cysteine rich domain, and three type III fibronectin repeats, remains unclear. The presence of fibronectin domains would predict the occurrence of heparin-binding sites, which may account for the documented affinity of CD45 for acid polysaccharides. We addressed this hypothesis using soluble recombinant proteins corresponding to the individual fibronectin domains (FN1 to FN3), and to the entire extracellular portion of CD45 (sCD45). Binding of these proteins to heparin was examined by frontal affinity chromatography. We found that while the sCD45 bound to heparin with Kd of 3.2 × 10-8 mol/l, the binding of FN2 and FN3 was somewhat weaker (Kd was 1.4 and 7.4 × 10-7 mol/l, respectively). The FN1 domain did not interact with heparin. Our results bring definitive evidence for the existence of binding sites for acid polysaccharides in the extracellular domain of CD45. These binding sites may be important for surface interactions of CD45 and for leukocyte signaling.
Interaction of heparin with fibronectin and isolated fibronectin domains