Exogenous receptor-associated protein binds to two distinct sites on human fibroblasts but does not bind to the glycosaminoglycan residues of heparan sulfate proteoglycans.

We have investigated the proposal that the receptor-associated protein (RAP) of the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor binds to heparan sulfate proteoglycans (HSP). 125I-RAP binds to two sites on the surface of fibroblasts as follows: a high affinity site with a Kd of 1.4 nM and a low affinity site (Kd = 188 nM) with a capacity of more than 1000-fold the maximum amount of lipoprotein receptor-related protein/alpha 2-macroglobulin receptor on the cell surface. 125I-RAP binding to the low affinity site was abolished by heparin or Suramin. However, maximal digestion of the glycosaminoglycan chains of HSP with heparinase or culturing the cells in chlorate, an inhibitor of proteoglycan sulfation, did not affect the binding of 125I-RAP or of 125I-labeled, methylamine-activated alpha 2-macroglobulin. Comparison of 125I-RAP degradation at two different concentrations suggests that the low affinity, high capacity site on the surface of human fibroblasts participates in the endocytosis of 125I-RAP. The nature of the low affinity site remains to be elucidated, but we can exclude the glycosaminoglycan chains of HSP.