Cation binding to the integrin CD11b I domain and activation model assessment

Background: The integrin family of cell-surface receptors mediate cell adhesion through interactions with the extracellular matrix or other cell-surface receptors. The α chain of some integrin heterodimers includes an inserted ‘I domain’ of about 200 amino acids which binds divalent metal ions and i...

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Bibliographic Details
Published in:Structure (London) 1998-07, Vol.6 (7), p.923-935
Main Authors: Baldwin, Eric T, Sarver, Ronald W, Bryant, Garold L, Curry, Kimberly A, Fairbanks, Michael B, Finzel, Barry C, Garlick, Robert L, Heinrikson, Robert L, Horton, Nancy C, Kelley, Laura-Lee C, Mildner, Ana M, Moon, Joseph B, Mott, John E, Mutchler, Veronica T, Tomich, Che-Shen C, Watenpaugh, Keith D, Wiley, Veronica H
Format: Article
Language:English
Subjects:
adhesion biology
Amino Acid Sequence
Binding Sites
Cadmium - chemistry
Cadmium - metabolism
calorimetry
Cations
crystallography
Crystallography, X-Ray
integrins
Macrophage-1 Antigen - chemistry
Macrophage-1 Antigen - metabolism
Magnesium - chemistry
Magnesium - metabolism
Manganese - chemistry
Manganese - metabolism
metal ions
Metals - chemistry
Metals - metabolism
Models, Molecular
Protein Conformation
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Summary:Background: The integrin family of cell-surface receptors mediate cell adhesion through interactions with the extracellular matrix or other cell-surface receptors. The α chain of some integrin heterodimers includes an inserted ‘I domain’ of about 200 amino acids which binds divalent metal ions and is essential for integrin function. Lee et al. proposed that the I domain of the integrin CD11b adopts a unique ‘active’ conformation when bound to its counter receptor. In addition, they proposed that the lack of adhesion in the presence of Ca 2+ ion reflected the stabilization of an ‘inactive’ I-domain conformation. We set out to independently determine the structure of the CD11b I domain and to evaluate the structural effects of divalent ion binding to this protein. Results: We have determined the X-ray structure of a new crystal form of the CD11b I domain in the absence of added metal ions by multiple isomorphous replacement (MIR). Metal ions were easily introduced into this crystal form allowing the straight-forward assessment of the structural effects of divalent cation binding at the metal ion dependent adhesion site (MIDAS). The equilibrium binding constants for these ions were determined by titration calorimetry. The overall protein conformation and metal-ion coordination of the I domain is the same as that observed for all previously reported CD11a I-domain structures and a CD11b I-domain complex with Mn 2+. These structures define a majority conformation. Conclusions: Addition of the cations Mg 2+, Mn 2+ and Cd 2+ to the metal-free I domain does not induce conformational changes in the crystalline environment. Moreover, we find that Ca 2+ binds poorly to the I domain which serves to explain its failure to support adhesion. We show that the active conformation proposed by Lee et al. is likely to be a construct artifact and we propose that the currently available data do not support a dramatic structural transition for the I domain during counter-receptor binding.
ISSN:0969-2126
1878-4186
DOI:10.1016/S0969-2126(98)00093-8