Regulation of integrin alpha 5 beta 1-fibronectin interactions by divalent cations. Evidence for distinct classes of binding sites for Mn2+, Mg2+, and Ca2+.
Integrin-ligand interactions are known to be dependent on divalent cations, although the precise role of cations in ligand binding is still unclear. Using the interaction between alpha 5 beta 1 and fibronectin as a model system, we have performed a comprehensive analysis of the effects of Mn2+, Mg2+, and Ca2+ on ligand binding. Each cation had distinct effects on the ligand-binding capacity of alpha 5 beta 1:Mn2+ promoted high levels of ligand binding, Mg2+ promoted low levels of binding, and Ca2+ failed to support binding. Studies of the effects of different combinations of cations on ligand binding indicated that the cation-binding sites within alpha 5 beta 1 are not all identical, or of broad specificity, but instead each site shows a distinct preference for one or more cations. Ca2+ strongly inhibited Mn(2+)-supported ligand binding, but this inhibition was noncompetitive, suggesting that Ca2+ recognizes different cation-binding sites to Mn2+. In contrast, Ca2+ acted as a direct competitive inhibitor of Mg(2+)-supported ligand binding, implying that Ca2+ can displace Mg2+ from the integrin. However, low concentrations of Ca2+ greatly increased the apparent affinity of Mg2+ for its binding site, suggesting the existence of a distinct high affinity Ca(2+)-binding site. Taken together, our results imply that the ligand-binding capacity of alpha 5 beta 1 can be regulated in a complex manner through separate classes of binding sites for Mn2+, Mg2+, and Ca2+.
Mould
A P
AP
Wellcome Trust Centre for Cell-Matrix Research, School of Biological Sciences, University of Manchester, United Kingdom.
Akiyama
S K
SK
Humphries
M J
MJ
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002413:Cations, Divalent / Q000378:metabolism / Q000494:pharmacology; D002448:Cell Adhesion; D002460:Cell Line; D002846:Chromatography, Affinity; D005260:Female; D005353:Fibronectins / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D004915:Leukemia, Erythroblastic, Acute; D007963:Leukocytes, Mononuclear / Q000502:physiology; D008024:Ligands; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D008345:Manganese / Q000378:metabolism / Q000494:pharmacology*; D010920:Placenta / Q000502:physiology; D011247:Pregnancy; D051381:Rats / Q000276:immunology; D017469:Receptors, Fibronectin / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D002413:Cations, Divalent; D005353:Fibronectins; D008024:Ligands; D017469:Receptors, Fibronectin; D008345:Manganese; D008274:Magnesium; D002118:Calcium