Regulation of the fibronectin receptor affinity by divalent cations.
The cell surface receptor for fibronectin is a heterodimeric membrane protein that recognizes an Arg-Gly-Asp sequence in fibronectin and that requires cations such as Mg2+ or Ca2+ for binding to fibronectin. The divalent cation requirements of this receptor were analyzed by measuring attachment of receptor liposomes to ligand-coated surfaces in the presence of different cations. The most striking effect observed was produced by Mn2+, which increased the binding of the receptor liposomes to fibronectin 2-3-fold over their binding in buffers containing Ca2+ and Mg2+. The binding activities of two related adhesion receptors, the vitronectin receptor and platelet GP IIb-IIIa, were supported but not enhanced by Mn2+. Two observations suggest that Mn2+ can compete with Ca2+ for the same cation-binding sites of the receptor. First, Mn2+ could still enhance fibronectin receptor binding activity even in the presence of 10-fold higher concentrations of Ca2+ or Mg2+. Second, Mn2+ inhibited the binding of radioactive Ca2+ to the alpha subunit of the receptor. The increased fibronectin receptor activity in the presence of Mn2+ appeared to be due to an increase in the affinity of the receptor for the Arg-Gly-Asp sequence because a 110-kDa cell attachment fragment and a synthetic hexapeptide containing the Arg-Gly-Asp sequence inhibited liposome binding more effectively in the presence of Mn2+ than in the presence of Ca2+/Mg2+. The affinity for the peptide was affected more than the affinity for the fragment, indicating that Mn2+ also induces a change in receptor specificity. Increased receptor binding in the presence of Mn2+ was also apparent in affinity chromatography of the fibronectin receptor on the 110-kDa fibronectin fragment; Mn2+ improved the yield of the receptor 4-fold. Mn2+ similarly increased the number of receptor-fibronectin complexes in preparations analyzed by electron microscopy. These results show that exogenous influences can modulate the affinity and specificity with which the fibronectin receptor binds to its ligands.
Gailit
J
J
Cancer Research Center, La Jolla Cancer Research Foundation, California 92037.
Ruoslahti
E
E
D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent / Q000494:pharmacology*; D006801:Humans; D008081:Liposomes / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D008854:Microscopy, Electron; D017469:Receptors, Fibronectin; D011971:Receptors, Immunologic / Q000378:metabolism*; D019030:Receptors, Vitronectin
D002413:Cations, Divalent; D008081:Liposomes; D017469:Receptors, Fibronectin; D011971:Receptors, Immunologic; D019030:Receptors, Vitronectin; D008345:Manganese; D008274:Magnesium; D002118:Calcium